diff --git a/Editor/API/AnimatorServices/AnimationIndex.cs b/Editor/API/AnimatorServices/AnimationIndex.cs
index 6bcbcb4..f298bfc 100644
--- a/Editor/API/AnimatorServices/AnimationIndex.cs
+++ b/Editor/API/AnimatorServices/AnimationIndex.cs
@@ -1,4 +1,6 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using UnityEditor;
diff --git a/Editor/API/AnimatorServices/AnimatorServicesContext.cs b/Editor/API/AnimatorServices/AnimatorServicesContext.cs
index 8eb9310..2c401e4 100644
--- a/Editor/API/AnimatorServices/AnimatorServicesContext.cs
+++ b/Editor/API/AnimatorServices/AnimatorServicesContext.cs
@@ -1,4 +1,6 @@
-using System.Collections.Generic;
+#nullable enable
+
+using System.Collections.Generic;
 using System.Linq;
 using UnityEngine;
 using VRC.SDK3.Avatars.Components;
@@ -9,14 +11,20 @@ public sealed class AnimatorServicesContext : IExtensionContext
     {
         private class LayerState
         {
-            internal RuntimeAnimatorController originalController;
-            internal VirtualAnimatorController virtualController;
-        }
+            internal readonly RuntimeAnimatorController? OriginalController;
+            internal VirtualAnimatorController? VirtualController;
 
-        private CloneContext _cloneContext;
+            public LayerState(RuntimeAnimatorController? originalController)
+            {
+                OriginalController = originalController;
+            }
+        }
+        
         private readonly Dictionary<object, LayerState> _layerStates = new();
 
-        private IPlatformAnimatorBindings _platformBindings;
+        // initialized on activate
+        private IPlatformAnimatorBindings? _platformBindings;
+        private CloneContext? _cloneContext;
 
         public void OnActivate(BuildContext context)
         {
@@ -33,20 +41,16 @@ public void OnActivate(BuildContext context)
 
             _cloneContext = new CloneContext(_platformBindings);
 
-            foreach (var (type, controller, isDefault) in _platformBindings.GetInnateControllers(root))
+            foreach (var (type, controller, _) in _platformBindings.GetInnateControllers(root))
             {
-                _layerStates[type] = new LayerState
-                {
-                    originalController = controller,
-                    virtualController = null
-                };
+                _layerStates[type] = new LayerState(controller);
 
-                // TEMP
+                // TEMP - force all layers to be processed
                 _ = this[type];
             }
         }
 
-        public VirtualAnimatorController this[object key]
+        public VirtualAnimatorController? this[object key]
         {
             get
             {
@@ -55,16 +59,16 @@ public VirtualAnimatorController this[object key]
                     return null;
                 }
 
-                if (state.virtualController == null)
+                if (state.VirtualController == null)
                 {
-                    state.virtualController = _cloneContext.Clone(state.originalController);
+                    state.VirtualController = _cloneContext!.Clone(state.OriginalController);
                 }
 
-                return state.virtualController;
+                return state.VirtualController;
             }
-            set => _layerStates[key] = new LayerState
+            set => _layerStates[key] = new LayerState(null)
             {
-                virtualController = value
+                VirtualController = value
             };
         }
 
@@ -74,13 +78,13 @@ public void OnDeactivate(BuildContext context)
 
             var commitContext = new CommitContext();
 
-            var controllers = _layerStates.Where(kvp => kvp.Value.virtualController != null)
+            var controllers = _layerStates.Where(kvp => kvp.Value.VirtualController != null)
                 .ToDictionary(
                     k => k.Key,
-                    v => (RuntimeAnimatorController)commitContext.CommitObject(v.Value.virtualController)
+                    v => (RuntimeAnimatorController)commitContext.CommitObject(v.Value.VirtualController!)
                 );
 
-            _platformBindings.CommitInnateControllers(root, controllers);
+            _platformBindings!.CommitInnateControllers(root, controllers);
         }
     }
 }
\ No newline at end of file
diff --git a/Editor/API/AnimatorServices/CloneContext.cs b/Editor/API/AnimatorServices/CloneContext.cs
index 8e15210..5618f1e 100644
--- a/Editor/API/AnimatorServices/CloneContext.cs
+++ b/Editor/API/AnimatorServices/CloneContext.cs
@@ -1,6 +1,9 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using UnityEditor.Animations;
 using UnityEngine;
 using Object = UnityEngine.Object;
@@ -61,12 +64,14 @@ public AnimationClip MapClipOnClone(AnimationClip clip)
 
             return clip;
         }
-        
-        public bool TryGetValue<T, U>(T key, out U value) where U: IDisposable
+
+        public bool TryGetValue<T, U>(T key, out U? value) where U : IDisposable
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
+            
             var rv = _clones.TryGetValue(key, out var tmp);
 
-            if (rv) value = (U)tmp;
+            if (rv) value = (U?)tmp;
             else value = default;
 
             return rv;
@@ -74,17 +79,18 @@ public bool TryGetValue<T, U>(T key, out U value) where U: IDisposable
 
         public void Add<T, U>(T key, U value) where U: IDisposable
         {
+            if (key == null) throw new ArgumentNullException(nameof(key));
             _clones.Add(key, value);
         }
 
-        private U GetOrClone<T, U>(T key, Func<CloneContext, T, U> clone) where U : class, IDisposable
+        private U? GetOrClone<T, U>(T? key, Func<CloneContext, T, U> clone) where U : class, IDisposable
         {
             try
             {
                 _cloneDepth++;
 
                 if (key == null || (key is Object obj && obj == null)) return null;
-                if (TryGetValue(key, out U value)) return value;
+                if (TryGetValue(key, out U? value)) return value;
                 value = clone(this, key);
                 _clones[key] = value;
                 return value;
@@ -124,49 +130,57 @@ internal int AllocateSingleVirtualLayer()
             return _nextVirtualLayer++;
         }
 
-        public VirtualAnimatorController Clone(RuntimeAnimatorController controller)
+        [return: NotNullIfNotNull("controller")]
+        public VirtualAnimatorController? Clone(RuntimeAnimatorController? controller)
         {
             using var _ = new ProfilerScope("Clone Animator Controller", controller);
             return GetOrClone(controller, VirtualAnimatorController.Clone);
         }
-        
-        public VirtualLayer Clone(AnimatorControllerLayer layer, int index)
+
+        [return: NotNullIfNotNull("layer")]
+        public VirtualLayer? Clone(AnimatorControllerLayer? layer, int index)
         {
             using var _ = new ProfilerScope("Clone Animator Layer");
-            return GetOrClone(layer, (ctx, obj) => VirtualLayer.Clone(ctx, obj, index));
+            return GetOrClone(layer, (ctx, obj) => VirtualLayer.Clone(ctx, obj, index)!);
         }
-        
-        public VirtualStateMachine Clone(AnimatorStateMachine stateMachine)
+
+        [return: NotNullIfNotNull("stateMachine")]
+        public VirtualStateMachine? Clone(AnimatorStateMachine? stateMachine)
         {
             using var _ = new ProfilerScope("Clone State Machine", stateMachine);
             return GetOrClone(stateMachine, VirtualStateMachine.Clone);
         }
 
-        public VirtualStateTransition Clone(AnimatorStateTransition transition)
+        [return: NotNullIfNotNull("transition")]
+        public VirtualStateTransition? Clone(AnimatorStateTransition? transition)
         {
             using var _ = new ProfilerScope("Clone State Transition", transition);
             return GetOrClone(transition, VirtualStateTransition.Clone);
         }
 
-        public VirtualTransition Clone(AnimatorTransition transition)
+        [return: NotNullIfNotNull("transition")]
+        public VirtualTransition? Clone(AnimatorTransition? transition)
         {
             using var _ = new ProfilerScope("Clone Transition", transition);
             return GetOrClone(transition, VirtualTransition.Clone);
         }
 
-        public VirtualState Clone(AnimatorState state)
+        [return: NotNullIfNotNull("state")]
+        public VirtualState? Clone(AnimatorState? state)
         {
             using var _ = new ProfilerScope("Clone State", state);
             return GetOrClone(state, VirtualState.Clone);
         }
 
-        public VirtualMotion Clone(Motion m)
+        [return: NotNullIfNotNull("m")]
+        public VirtualMotion? Clone(Motion? m)
         {
             using var _ = new ProfilerScope("Clone Motion", m);
             return GetOrClone(m, VirtualMotion.Clone);
         }
 
-        public VirtualClip Clone(AnimationClip clip)
+        [return: NotNullIfNotNull("clip")]
+        public VirtualClip? Clone(AnimationClip? clip)
         {
             using var _ = new ProfilerScope("Clone Clip", clip);
             return GetOrClone(clip, VirtualClip.Clone);
diff --git a/Editor/API/AnimatorServices/ECBComparator.cs b/Editor/API/AnimatorServices/ECBComparator.cs
index 9ace0a6..78528b9 100644
--- a/Editor/API/AnimatorServices/ECBComparator.cs
+++ b/Editor/API/AnimatorServices/ECBComparator.cs
@@ -1,4 +1,6 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using UnityEditor;
 
diff --git a/Editor/API/AnimatorServices/ICommitable.cs b/Editor/API/AnimatorServices/ICommitable.cs
index 8c85632..501648a 100644
--- a/Editor/API/AnimatorServices/ICommitable.cs
+++ b/Editor/API/AnimatorServices/ICommitable.cs
@@ -1,4 +1,7 @@
-using System.Collections.Generic;
+#nullable enable
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using UnityEngine;
 
 namespace nadena.dev.ndmf.animator
@@ -16,6 +19,7 @@ internal interface ICommitable<T>
         ///     Fills in all fields of the destination unity object. This may recurse back into the CommitContext.
         /// </summary>
         /// <param name="context"></param>
+        /// <param name="obj">Object returned from Prepare</param>
         void Commit(CommitContext context, T obj);
     }
 
@@ -25,7 +29,8 @@ internal class CommitContext
         private readonly Dictionary<int, VirtualLayer> _virtIndexToVirtLayer = new();
         private readonly Dictionary<VirtualLayer, int> _virtLayerToPhysIndex = new();
 
-        internal R CommitObject<R>(ICommitable<R> obj) where R : class
+        [return: NotNullIfNotNull("obj")]
+        internal R? CommitObject<R>(ICommitable<R>? obj) where R : class
         {
             if (obj == null) return null;
             if (_commitCache.TryGetValue(obj, out var result)) return (R)result;
diff --git a/Editor/API/AnimatorServices/LayerPriority.cs b/Editor/API/AnimatorServices/LayerPriority.cs
index 2221cfc..1f574a3 100644
--- a/Editor/API/AnimatorServices/LayerPriority.cs
+++ b/Editor/API/AnimatorServices/LayerPriority.cs
@@ -1,21 +1,23 @@
-using System;
+#nullable enable
+
+using System;
 
 namespace nadena.dev.ndmf.animator
 {
-    public struct LayerPriority : IComparable<LayerPriority>
+    public struct LayerPriority : IComparable<LayerPriority>, IEquatable<LayerPriority>
     {
         public static LayerPriority Default = new();
-        
-        internal int Priority;
+
+        private readonly int _priority;
 
         public LayerPriority(int priority)
         {
-            Priority = priority;
+            _priority = priority;
         }
 
         public int CompareTo(LayerPriority other)
         {
-            if (Priority != other.Priority) return Priority.CompareTo(other.Priority);
+            if (_priority != other._priority) return _priority.CompareTo(other._priority);
 
             return 0;
         }
@@ -52,12 +54,17 @@ public int CompareTo(LayerPriority other)
 
         public override bool Equals(object obj)
         {
-            return obj is LayerPriority other && this == other;
+            return obj is LayerPriority other && Equals(other);
         }
 
         public override int GetHashCode()
         {
-            return Priority.GetHashCode();
+            return _priority;
+        }
+
+        public bool Equals(LayerPriority other)
+        {
+            return _priority == other._priority;
         }
     }
 }
\ No newline at end of file
diff --git a/Editor/API/AnimatorServices/ObjectPathRemapper.cs b/Editor/API/AnimatorServices/ObjectPathRemapper.cs
index 5546bdf..64dc6d9 100644
--- a/Editor/API/AnimatorServices/ObjectPathRemapper.cs
+++ b/Editor/API/AnimatorServices/ObjectPathRemapper.cs
@@ -1,4 +1,6 @@
-using System.Collections.Generic;
+#nullable enable
+
+using System.Collections.Generic;
 using nadena.dev.ndmf.runtime;
 using UnityEngine;
 
@@ -53,9 +55,13 @@ private void UpdateCache()
 
             foreach (var kvp in _objectToOriginalPaths)
             {
+                var virtualPath = GetVirtualPathForObject(kvp.Key);
+
+                if (virtualPath == null) continue;
+                
                 foreach (var path in kvp.Value)
                 {
-                    _originalToMappedPath[path] = GetVirtualPathForObject(kvp.Key);
+                    _originalToMappedPath[path] = virtualPath;
                 }
             }
         }
@@ -70,18 +76,18 @@ public void RecordObjectTree(Transform subtree)
             }
         }
 
-        public GameObject GetObjectForPath(string path)
+        public GameObject? GetObjectForPath(string path)
         {
             var xform = _pathToObject.GetValueOrDefault(path);
             return xform ? xform.gameObject : null;
         }
 
-        public string GetVirtualPathForObject(GameObject obj)
+        public string? GetVirtualPathForObject(GameObject obj)
         {
             return GetVirtualPathForObject(obj.transform);
         }
 
-        public string GetVirtualPathForObject(Transform t)
+        public string? GetVirtualPathForObject(Transform t)
         {
             if (_objectToOriginalPaths.TryGetValue(t, out var paths))
             {
diff --git a/Editor/API/AnimatorServices/VirtualObjects/SyncedLayerOverrideAccess.cs b/Editor/API/AnimatorServices/VirtualObjects/SyncedLayerOverrideAccess.cs
index 36fd2e5..27cff2f 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/SyncedLayerOverrideAccess.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/SyncedLayerOverrideAccess.cs
@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
@@ -17,14 +18,14 @@ namespace nadena.dev.ndmf.animator
     [SuppressMessage("ReSharper", "InconsistentNaming")]
     internal class SyncedLayerOverrideAccess
     {
-        public static readonly Func<AnimatorControllerLayer, IEnumerable<KeyValuePair<AnimatorState, Motion>>>
+        public static readonly Func<AnimatorControllerLayer, IEnumerable<KeyValuePair<AnimatorState, Motion>>?>
             ExtractStateMotionPairs;
 
         public static readonly Action<AnimatorControllerLayer, IEnumerable<KeyValuePair<AnimatorState, Motion>>>
             SetStateMotionPairs;
 
         public static readonly
-            Func<AnimatorControllerLayer, IEnumerable<KeyValuePair<AnimatorState, ScriptableObject[]>>>
+            Func<AnimatorControllerLayer, IEnumerable<KeyValuePair<AnimatorState, ScriptableObject[]>>?>
             ExtractStateBehaviourPairs;
 
         public static readonly
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualAnimatorController.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualAnimatorController.cs
index aee1e4e..22b725c 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualAnimatorController.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualAnimatorController.cs
@@ -1,4 +1,6 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -41,8 +43,8 @@ private struct LayerGroup
         public VirtualAnimatorController(CloneContext context, string name = "")
         {
             _context = context;
+            _parameters = ImmutableDictionary<string, AnimatorControllerParameter>.Empty;
             Name = name;
-            Parameters = ImmutableDictionary<string, AnimatorControllerParameter>.Empty;
         }
 
         public void AddLayer(LayerPriority priority, VirtualLayer layer)
@@ -73,7 +75,7 @@ public IEnumerable<VirtualLayer> Layers
             get { return _layers.Values.SelectMany(l => l.Layers); }
         }
 
-        public static VirtualAnimatorController Clone(CloneContext context, RuntimeAnimatorController controller)
+        internal static VirtualAnimatorController Clone(CloneContext context, RuntimeAnimatorController controller)
         {
             switch (controller)
             {
@@ -90,8 +92,9 @@ public static VirtualAnimatorController Clone(CloneContext context, RuntimeAnima
 
         private VirtualAnimatorController(CloneContext context, AnimatorController controller)
         {
+            _context = context;
             Name = controller.name;
-            Parameters = controller.parameters.ToImmutableDictionary(p => p.name);
+            _parameters = controller.parameters.ToImmutableDictionary(p => p.name);
 
             var srcLayers = controller.layers;
             context.AllocateVirtualLayerSpace(srcLayers.Length);
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualBlendTree.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualBlendTree.cs
index cee2dc0..a19180f 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualBlendTree.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualBlendTree.cs
@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+#nullable enable
+using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
 using UnityEditor;
@@ -10,13 +11,12 @@ namespace nadena.dev.ndmf.animator
     public class VirtualBlendTree : VirtualMotion
     {
         private readonly BlendTree _tree;
-        private VirtualChildMotion _motions;
 
         public sealed class VirtualChildMotion
         {
-            public VirtualMotion Motion;
+            public VirtualMotion? Motion;
             public float CycleOffset;
-            public string DirectBlendParameter;
+            public string DirectBlendParameter = "Blend";
             public bool Mirror;
             public float Threshold;
             public Vector2 Position;
@@ -26,32 +26,29 @@ public sealed class VirtualChildMotion
         private VirtualBlendTree(CloneContext context, BlendTree cloned)
         {
             _tree = cloned;
+            _children = ImmutableList<VirtualChildMotion>.Empty;
 
             context.DeferCall(() =>
             {
-                Children = _tree.children.Select(m =>
+                Children = _tree.children.Select(m => new VirtualChildMotion
                 {
-                    return new VirtualChildMotion
-                    {
-                        Motion = context.Clone(m.motion),
-                        CycleOffset = m.cycleOffset,
-                        DirectBlendParameter = m.directBlendParameter,
-                        Mirror = m.mirror,
-                        Threshold = m.threshold,
-                        Position = m.position,
-                        TimeScale = m.timeScale
-                    };
+                    Motion = context.Clone(m.motion),
+                    CycleOffset = m.cycleOffset,
+                    DirectBlendParameter = m.directBlendParameter,
+                    Mirror = m.mirror,
+                    Threshold = m.threshold,
+                    Position = m.position,
+                    TimeScale = m.timeScale
                 }).ToImmutableList();
             });
         }
 
-        public static VirtualBlendTree Clone(
+        internal static VirtualBlendTree Clone(
             CloneContext context,
             BlendTree tree
         )
         {
-            if (tree == null) return null;
-            if (context.TryGetValue(tree, out VirtualBlendTree existing)) return existing;
+            if (context.TryGetValue(tree, out VirtualBlendTree? existing)) return existing!;
 
             var cloned = new BlendTree();
             EditorUtility.CopySerialized(tree, cloned);
@@ -142,7 +139,7 @@ public override void Dispose()
 
         protected override IEnumerable<VirtualNode> _EnumerateChildren()
         {
-            return Children.Select(c => c.Motion);
+            return Children.Where(c => c.Motion != null).Select(c => c.Motion!);
         }
     }
 }
\ No newline at end of file
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualClip.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualClip.cs
index 0c17f1b..86f9125 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualClip.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualClip.cs
@@ -1,10 +1,12 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using JetBrains.Annotations;
 using UnityEditor;
 using UnityEngine;
-using Debug = System.Diagnostics.Debug;
 using Object = UnityEngine.Object;
 
 namespace nadena.dev.ndmf.animator
@@ -75,9 +77,9 @@ public AnimationClipSettings Settings
             }
         }
 
-        private VirtualMotion _additiveReferencePoseClip;
+        private VirtualMotion? _additiveReferencePoseClip;
 
-        public VirtualMotion AdditiveReferencePoseClip
+        public VirtualMotion? AdditiveReferencePoseClip
         {
             get => _additiveReferencePoseClip;
             set => _additiveReferencePoseClip = I(value);
@@ -115,7 +117,7 @@ private struct CachedCurve<T>
         {
             // If null and Dirty is false, the curve has not been cached yet.
             // If null and Dirty is true, the curve has been deleted.
-            public T Value;
+            public T? Value;
             public bool Dirty;
 
             public override string ToString()
@@ -140,7 +142,7 @@ public static VirtualClip FromMarker(AnimationClip clip)
         ///     Clones an animation clip into a VirtualClip. The provided BuildContext is used to determine which platform
         ///     to use to query for marker clips; if a marker clip is found, it will be treated as immutable.
         /// </summary>
-        /// <param name="context"></param>
+        /// <param name="cloneContext"></param>
         /// <param name="clip"></param>
         /// <returns></returns>
         public static VirtualClip Clone(
@@ -148,8 +150,6 @@ public static VirtualClip Clone(
             AnimationClip clip
         )
         {
-            if (clip == null) return null;
-
             clip = cloneContext.MapClipOnClone(clip);
 
             if (cloneContext.PlatformBindings.IsSpecialMotion(clip))
@@ -157,11 +157,9 @@ AnimationClip clip
                 return FromMarker(clip);
             }
 
-            // workaround Rider warning bug
-            VirtualClip clonedClip = null;
-            if (cloneContext?.TryGetValue(clip, out clonedClip) == true)
+            if (cloneContext.TryGetValue(clip, out VirtualClip? clonedClip))
             {
-                return clonedClip;
+                return clonedClip!;
             }
 
             var newClip = Object.Instantiate(clip);
@@ -320,7 +318,7 @@ public AnimationCurve GetFloatCurve(EditorCurveBinding binding)
                 }
             }
 
-            return cached.Value;
+            return cached.Value!;
         }
 
         public ObjectReferenceKeyframe[] GetObjectCurve(EditorCurveBinding binding)
@@ -334,7 +332,7 @@ public ObjectReferenceKeyframe[] GetObjectCurve(EditorCurveBinding binding)
                 }
             }
 
-            return cached.Value;
+            return cached.Value!;
         }
 
         public void SetFloatCurve(EditorCurveBinding binding, AnimationCurve curve)
@@ -388,7 +386,11 @@ protected override Motion Prepare(object context)
             return _clip;
         }
 
-        protected override void Commit(object context_, Motion obj)
+        protected override void Commit(
+            [SuppressMessage("ReSharper", "InconsistentNaming")]
+            object context_,
+            Motion obj
+        )
         {
             if (IsMarkerClip || !IsDirty) return;
             
@@ -441,8 +443,6 @@ protected override void Commit(object context_, Motion obj)
                 : null;
             settings.additiveReferencePoseTime = AdditiveReferencePoseTime;
             AnimationUtility.SetAnimationClipSettings(clip, settings);
-            
-            this._clip = null;
         }
 
         public AnimationCurve GetFloatCurve(string path, Type type, string prop)
@@ -468,7 +468,6 @@ public void SetObjectCurve(string path, Type type, string prop, ObjectReferenceK
         public override void Dispose()
         {
             if (_clip != null) Object.DestroyImmediate(_clip);
-            this._clip = null;
         }
     }
 }
\ No newline at end of file
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualLayer.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualLayer.cs
index 0b78a4e..8413202 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualLayer.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualLayer.cs
@@ -1,7 +1,10 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
+using JetBrains.Annotations;
 using UnityEditor.Animations;
 using UnityEngine;
 using Object = UnityEngine.Object;
@@ -11,6 +14,7 @@ namespace nadena.dev.ndmf.animator
     /// <summary>
     ///     A layer within a VirtualAnimatorController
     /// </summary>
+    [PublicAPI]
     public class VirtualLayer : VirtualNode, ICommitable<AnimatorControllerLayer>, IDisposable
     {
         /// <summary>
@@ -18,8 +22,7 @@ public class VirtualLayer : VirtualNode, ICommitable<AnimatorControllerLayer>, I
         ///     even if layer order changes. This will typically be a very large value (>2^16).
         /// </summary>
         public int VirtualLayerIndex { get; }
-
-
+        
         private VirtualStateMachine _stateMachine;
 
         public VirtualStateMachine StateMachine
@@ -28,9 +31,9 @@ public VirtualStateMachine StateMachine
             set => _stateMachine = I(value);
         }
 
-        private AvatarMask _avatarMask;
+        private AvatarMask? _avatarMask;
 
-        public AvatarMask AvatarMask
+        public AvatarMask? AvatarMask
         {
             get => _avatarMask;
             set => _avatarMask = I(value);
@@ -108,14 +111,10 @@ public ImmutableDictionary<VirtualState, ImmutableList<StateMachineBehaviour>> S
             set => _syncedLayerBehaviourOverrides = I(value);
         }
 
-        public static VirtualLayer Clone(CloneContext context, AnimatorControllerLayer layer, int physicalLayerIndex)
+        internal static VirtualLayer Clone(CloneContext context, AnimatorControllerLayer layer, int physicalLayerIndex)
         {
-            if (layer == null) return null;
-
             var clone = new VirtualLayer(context, layer, physicalLayerIndex);
 
-            // TODO: motion, behavior overrides
-
             return clone;
         }
 
@@ -127,7 +126,7 @@ public static VirtualLayer Create(CloneContext context, string name = "(unnamed)
         private VirtualLayer(CloneContext context, AnimatorControllerLayer layer, int physicalLayerIndex)
         {
             VirtualLayerIndex = context.CloneSourceToVirtualLayerIndex(physicalLayerIndex);
-            Name = layer.name;
+            _name = layer.name;
             AvatarMask = layer.avatarMask == null ? null : Object.Instantiate(layer.avatarMask);
             BlendingMode = layer.blendingMode;
             DefaultWeight = layer.defaultWeight;
@@ -135,21 +134,25 @@ private VirtualLayer(CloneContext context, AnimatorControllerLayer layer, int ph
             SyncedLayerAffectsTiming = layer.syncedLayerAffectsTiming;
             SyncedLayerIndex = context.CloneSourceToVirtualLayerIndex(layer.syncedLayerIndex);
 
-            StateMachine = VirtualStateMachine.Clone(context, layer.stateMachine);
+            _stateMachine = context.Clone(layer.stateMachine);
 
             _syncedLayerMotionOverrides = SyncedLayerOverrideAccess.ExtractStateMotionPairs(layer)
-                ?.ToImmutableDictionary(kvp => context.Clone(kvp.Key), kvp => context.Clone(kvp.Value));
+                                              ?.ToImmutableDictionary(kvp => context.Clone(kvp.Key),
+                                                  kvp => context.Clone(kvp.Value))
+                                          ?? ImmutableDictionary<VirtualState, VirtualMotion>.Empty;
 
             // TODO: Apply state behavior import processing
             _syncedLayerBehaviourOverrides = SyncedLayerOverrideAccess.ExtractStateBehaviourPairs(layer)
                 ?.ToImmutableDictionary(kvp => context.Clone(kvp.Key),
-                    kvp => kvp.Value.Cast<StateMachineBehaviour>().ToImmutableList());
+                    kvp => kvp.Value.Cast<StateMachineBehaviour>().ToImmutableList())
+                                             ?? ImmutableDictionary<VirtualState, ImmutableList<StateMachineBehaviour>>
+                                                 .Empty;
         }
 
         private VirtualLayer(CloneContext context, string name)
         {
             VirtualLayerIndex = context.AllocateSingleVirtualLayer();
-            Name = name;
+            _name = name;
             AvatarMask = null;
             BlendingMode = AnimatorLayerBlendingMode.Override;
             DefaultWeight = 1;
@@ -157,7 +160,10 @@ private VirtualLayer(CloneContext context, string name)
             SyncedLayerAffectsTiming = false;
             SyncedLayerIndex = -1;
 
-            StateMachine = VirtualStateMachine.Create(context, name);
+            _stateMachine = VirtualStateMachine.Create(context, name);
+            _syncedLayerMotionOverrides = ImmutableDictionary<VirtualState, VirtualMotion>.Empty;
+            _syncedLayerBehaviourOverrides =
+                ImmutableDictionary<VirtualState, ImmutableList<StateMachineBehaviour>>.Empty;
         }
 
         AnimatorControllerLayer ICommitable<AnimatorControllerLayer>.Prepare(CommitContext context)
@@ -182,19 +188,14 @@ void ICommitable<AnimatorControllerLayer>.Commit(CommitContext context, Animator
             obj.syncedLayerIndex = context.VirtualToPhysicalLayerIndex(SyncedLayerIndex);
             obj.stateMachine = context.CommitObject(StateMachine);
 
-            var motionOverrides = SyncedLayerMotionOverrides ?? ImmutableDictionary<VirtualState, VirtualMotion>.Empty;
-
-            SyncedLayerOverrideAccess.SetStateMotionPairs(obj, motionOverrides.Select(kvp =>
+            SyncedLayerOverrideAccess.SetStateMotionPairs(obj, SyncedLayerMotionOverrides.Select(kvp =>
                 new KeyValuePair<AnimatorState, Motion>(
                     context.CommitObject(kvp.Key),
                     context.CommitObject(kvp.Value)
                 )));
 
-            var behaviourOverrides = SyncedLayerBehaviourOverrides ??
-                                     ImmutableDictionary<VirtualState, ImmutableList<StateMachineBehaviour>>.Empty;
-
             // TODO: commit state behaviours
-            SyncedLayerOverrideAccess.SetStateBehaviourPairs(obj, behaviourOverrides.Select(kvp =>
+            SyncedLayerOverrideAccess.SetStateBehaviourPairs(obj, SyncedLayerBehaviourOverrides.Select(kvp =>
                 new KeyValuePair<AnimatorState, ScriptableObject[]>(
                     context.CommitObject(kvp.Key),
                     kvp.Value.Cast<ScriptableObject>().ToArray()
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualMotion.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualMotion.cs
index 9a64c28..4eb9b4e 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualMotion.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualMotion.cs
@@ -1,4 +1,6 @@
-using System;
+#nullable enable
+
+using System;
 using UnityEditor.Animations;
 using UnityEngine;
 
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualNode.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualNode.cs
index 87c6ea0..072b53c 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualNode.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualNode.cs
@@ -1,4 +1,6 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 
 namespace nadena.dev.ndmf.animator
@@ -6,7 +8,7 @@ namespace nadena.dev.ndmf.animator
     [ExcludeFromDocs]
     public abstract class VirtualNode
     {
-        private Action _lastCacheObserver;
+        private Action? _lastCacheObserver;
 
         internal VirtualNode()
         {
@@ -24,7 +26,7 @@ internal T I<T>(T val)
             return val;
         }
 
-        internal void RegisterCacheObserver(Action observer)
+        internal void RegisterCacheObserver(Action? observer)
         {
             if (observer != _lastCacheObserver && _lastCacheObserver != null)
             {
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualState.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualState.cs
index f9b66dc..96cef58 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualState.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualState.cs
@@ -1,4 +1,6 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -10,11 +12,12 @@
 
 namespace nadena.dev.ndmf.animator
 {
+    [PublicAPI]
     public class VirtualState : VirtualNode, ICommitable<AnimatorState>, IDisposable
     {
         private AnimatorState _state;
 
-        private ImmutableList<StateMachineBehaviour> _behaviours;
+        private ImmutableList<StateMachineBehaviour> _behaviours = ImmutableList<StateMachineBehaviour>.Empty;
 
         public ImmutableList<StateMachineBehaviour> Behaviours
         {
@@ -27,9 +30,7 @@ public static VirtualState Clone(
             AnimatorState state
         )
         {
-            if (state == null) return null;
-
-            if (context.TryGetValue(state, out VirtualState clone)) return clone;
+            if (context.TryGetValue(state, out VirtualState? clone)) return clone!;
 
             var clonedState = new AnimatorState();
             // We can't use Instantiate for AnimatorStates, for some reason...
@@ -47,7 +48,7 @@ private VirtualState()
         {
             _state = new AnimatorState();
             Behaviours = ImmutableList<StateMachineBehaviour>.Empty;
-            Transitions = ImmutableList<VirtualStateTransition>.Empty;
+            _transitions = ImmutableList<VirtualStateTransition>.Empty;
         }
 
         private VirtualState(CloneContext context, AnimatorState clonedState)
@@ -56,13 +57,22 @@ private VirtualState(CloneContext context, AnimatorState clonedState)
 
             // TODO: Should we rewrite any internal properties of these StateMachineBehaviours?
             Behaviours = _state.behaviours.Select(b => Object.Instantiate(b)).ToImmutableList();
-            context.DeferCall(() => { Transitions = _state.transitions.Select(context.Clone).ToImmutableList(); });
+
+            _transitions = ImmutableList<VirtualStateTransition>.Empty;
+            context.DeferCall(() =>
+            {
+                Transitions = _state.transitions
+                    .Where(t => t != null)
+                    .Select(context.Clone)
+                    .ToImmutableList()!;
+            });
+            
             Motion = context.Clone(_state.motion);
         }
 
-        private VirtualMotion _motion;
+        private VirtualMotion? _motion;
 
-        public VirtualMotion Motion
+        public VirtualMotion? Motion
         {
             get => _motion;
             set => _motion = I(value);
@@ -80,8 +90,7 @@ public float CycleOffset
             set => _state.cycleOffset = I(value);
         }
 
-        [CanBeNull]
-        public string CycleOffsetParameter
+        public string? CycleOffsetParameter
         {
             get => _state.cycleOffsetParameterActive ? _state.cycleOffsetParameter : null;
             set
@@ -104,8 +113,7 @@ public bool Mirror
             set => _state.mirror = I(value);
         }
 
-        [CanBeNull]
-        public string MirrorParameter
+        public string? MirrorParameter
         {
             get => _state.mirrorParameterActive ? _state.mirrorParameter : null;
             set
@@ -124,8 +132,7 @@ public float Speed
             set => _state.speed = I(value);
         }
 
-        [CanBeNull]
-        public string SpeedParameter
+        public string? SpeedParameter
         {
             get => _state.speedParameterActive ? _state.speedParameter : null;
             set
@@ -142,8 +149,7 @@ public string Tag
             set => _state.tag = I(value);
         }
 
-        [CanBeNull]
-        public string TimeParameter
+        public string? TimeParameter
         {
             get => _state.timeParameterActive ? _state.timeParameter : null;
             set
@@ -192,11 +198,10 @@ void IDisposable.Dispose()
             {
                 Object.DestroyImmediate(behaviour);
             }
-            
-            Behaviours.Clear();
+
+            Behaviours = Behaviours.Clear();
             
             if (_state != null) Object.DestroyImmediate(_state);
-            _state = null;
         }
 
         public override string ToString()
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualStateMachine.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualStateMachine.cs
index 3e39ab0..f835c1d 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualStateMachine.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualStateMachine.cs
@@ -1,8 +1,9 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
-using JetBrains.Annotations;
 using UnityEditor.Animations;
 using UnityEngine;
 using Object = UnityEngine.Object;
@@ -19,8 +20,7 @@ public class VirtualStateMachine : VirtualNode, ICommitable<AnimatorStateMachine
 
         public static VirtualStateMachine Clone(CloneContext context, AnimatorStateMachine stateMachine)
         {
-            if (stateMachine == null) return null;
-            if (context.TryGetValue(stateMachine, out VirtualStateMachine clone)) return clone;
+            if (context.TryGetValue(stateMachine, out VirtualStateMachine? clone)) return clone!;
 
             var vsm = new VirtualStateMachine(context, stateMachine.name);
 
@@ -28,33 +28,44 @@ public static VirtualStateMachine Clone(CloneContext context, AnimatorStateMachi
             {
                 vsm.AnyStatePosition = stateMachine.anyStatePosition;
                 vsm.AnyStateTransitions = stateMachine.anyStateTransitions
-                    .Select(context.Clone).ToImmutableList();
+                    .Where(t => t != null)
+                    .Select(context.Clone)
+                    .ToImmutableList()!;
                 vsm.Behaviours = stateMachine.behaviours.Select(Object.Instantiate).ToImmutableList();
                 vsm.DefaultState = context.Clone(stateMachine.defaultState);
                 vsm.EntryPosition = stateMachine.entryPosition;
                 vsm.EntryTransitions = stateMachine.entryTransitions
-                    .Select(context.Clone).ToImmutableList();
+                    .Where(t => t != null)
+                    .Select(context.Clone)
+                    .ToImmutableList()!;
                 vsm.ExitPosition = stateMachine.exitPosition;
                 vsm.ParentStateMachinePosition = stateMachine.parentStateMachinePosition;
 
-                vsm.StateMachines = stateMachine.stateMachines.Select(sm => new VirtualChildStateMachine
-                {
-                    StateMachine = context.Clone(sm.stateMachine),
-                    Position = sm.position
-                }).ToImmutableList();
-
-                vsm.States = stateMachine.states.Select(s => new VirtualChildState
-                {
-                    State = context.Clone(s.state),
-                    Position = s.position
-                }).ToImmutableList();
+                vsm.StateMachines = stateMachine.stateMachines
+                    .Where(sm => sm.stateMachine != null)
+                    .Select(sm => new VirtualChildStateMachine
+                    {
+                        StateMachine = context.Clone(sm.stateMachine),
+                        Position = sm.position
+                    }).ToImmutableList();
+
+                vsm.States = stateMachine.states
+                    .Where(s => s.state != null)
+                    .Select(s => new VirtualChildState
+                    {
+                        State = context.Clone(s.state),
+                        Position = s.position
+                    }).ToImmutableList();
 
                 vsm.StateMachineTransitions = stateMachine.stateMachines
+                    .Where(sm => sm.stateMachine != null)
                     .ToImmutableDictionary(
                         sm => context.Clone(sm.stateMachine),
                         sm => stateMachine.GetStateMachineTransitions(sm.stateMachine)
-                            .Select(context.Clone).ToImmutableList()
-                    );
+                            .Where(t => t != null)
+                            .Select(context.Clone)
+                            .ToImmutableList()
+                    )!;
             });
 
             return vsm;
@@ -69,24 +80,22 @@ private VirtualStateMachine(CloneContext context, string name = "")
         {
             _context = context;
             _stateMachine = new AnimatorStateMachine();
-            Name = name;
+            _name = name;
             AnyStatePosition = _stateMachine.anyStatePosition;
             EntryPosition = _stateMachine.entryPosition;
             ExitPosition = _stateMachine.exitPosition;
 
-            EntryTransitions = ImmutableList<VirtualTransition>.Empty;
-            AnyStateTransitions = ImmutableList<VirtualStateTransition>.Empty;
-            Behaviours = ImmutableList<StateMachineBehaviour>.Empty;
-            StateMachines = ImmutableList<VirtualChildStateMachine>.Empty;
-            States = ImmutableList<VirtualChildState>.Empty;
-            StateMachineTransitions = ImmutableDictionary<VirtualStateMachine, ImmutableList<VirtualTransition>>.Empty;
+            _entryTransitions = ImmutableList<VirtualTransition>.Empty;
+            _anyStateTransitions = ImmutableList<VirtualStateTransition>.Empty;
+            _behaviours = ImmutableList<StateMachineBehaviour>.Empty;
+            _stateMachines = ImmutableList<VirtualChildStateMachine>.Empty;
+            _states = ImmutableList<VirtualChildState>.Empty;
+            _stateMachineTransitions = ImmutableDictionary<VirtualStateMachine, ImmutableList<VirtualTransition>>.Empty;
         }
         
         AnimatorStateMachine ICommitable<AnimatorStateMachine>.Prepare(CommitContext context)
         {
-            var tmp = _stateMachine;
-            _stateMachine = null;
-            return tmp;
+            return _stateMachine;
         }
 
         void ICommitable<AnimatorStateMachine>.Commit(CommitContext context, AnimatorStateMachine obj)
@@ -214,9 +223,9 @@ public ImmutableDictionary<VirtualStateMachine, ImmutableList<VirtualTransition>
             set => _stateMachineTransitions = I(value);
         }
 
-        private VirtualState _defaultState;
+        private VirtualState? _defaultState;
 
-        public VirtualState DefaultState
+        public VirtualState? DefaultState
         {
             get => _defaultState;
             set => _defaultState = I(value);
@@ -239,7 +248,6 @@ public void Dispose()
             if (_stateMachine != null)
             {
                 Object.DestroyImmediate(_stateMachine);
-                _stateMachine = null;
             }
         }
 
@@ -268,7 +276,7 @@ protected override IEnumerable<VirtualNode> _EnumerateChildren()
             }
         }
 
-        public VirtualState AddState(string name, [CanBeNull] VirtualMotion motion = null, Vector3? position = null)
+        public VirtualState AddState(string name, VirtualMotion? motion = null, Vector3? position = null)
         {
             var state = VirtualState.Create(_context, name);
 
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualStateTransition.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualStateTransition.cs
index b3ffa52..e6e17bb 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualStateTransition.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualStateTransition.cs
@@ -1,7 +1,11 @@
-using UnityEditor.Animations;
+#nullable enable
+
+using JetBrains.Annotations;
+using UnityEditor.Animations;
 
 namespace nadena.dev.ndmf.animator
 {
+    [PublicAPI]
     public class VirtualStateTransition : VirtualTransitionBase
     {
         private readonly AnimatorStateTransition _stateTransition;
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualTransition.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualTransition.cs
index ca06db4..311b7d3 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualTransition.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualTransition.cs
@@ -1,4 +1,6 @@
-using UnityEditor.Animations;
+#nullable enable
+
+using UnityEditor.Animations;
 
 namespace nadena.dev.ndmf.animator
 {
diff --git a/Editor/API/AnimatorServices/VirtualObjects/VirtualTransitionBase.cs b/Editor/API/AnimatorServices/VirtualObjects/VirtualTransitionBase.cs
index a41de2e..1604ce3 100644
--- a/Editor/API/AnimatorServices/VirtualObjects/VirtualTransitionBase.cs
+++ b/Editor/API/AnimatorServices/VirtualObjects/VirtualTransitionBase.cs
@@ -1,17 +1,23 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using JetBrains.Annotations;
 using UnityEditor.Animations;
 using Object = UnityEngine.Object;
 
 namespace nadena.dev.ndmf.animator
 {
+    [PublicAPI]
     public class VirtualTransitionBase : VirtualNode, ICommitable<AnimatorTransitionBase>, IDisposable
     {
         protected AnimatorTransitionBase _transition;
-        private ImmutableList<AnimatorCondition> _conditions;
 
-        internal VirtualTransitionBase(CloneContext context, AnimatorTransitionBase cloned)
+        // null indicates we've deferred reading the conditions from the transition object
+        private ImmutableList<AnimatorCondition>? _conditions;
+
+        internal VirtualTransitionBase(CloneContext? context, AnimatorTransitionBase cloned)
         {
             _transition = cloned;
 
@@ -53,9 +59,9 @@ public ImmutableList<AnimatorCondition> Conditions
             }
         }
 
-        private VirtualState _destinationState;
+        private VirtualState? _destinationState;
 
-        public VirtualState DestinationState
+        public VirtualState? DestinationState
         {
             get => _destinationState;
             private set
@@ -65,9 +71,9 @@ private set
             }
         }
 
-        private VirtualStateMachine _destinationStateMachine;
+        private VirtualStateMachine? _destinationStateMachine;
 
-        public VirtualStateMachine DestinationStateMachine
+        public VirtualStateMachine? DestinationStateMachine
         {
             get => _destinationStateMachine;
             private set
@@ -95,11 +101,9 @@ protected static VirtualTransitionBase CloneInternal(
             AnimatorTransitionBase transition
         )
         {
-            if (transition == null) return null;
+            if (context.TryGetValue(transition, out VirtualStateTransition? clone)) return clone!;
 
-            if (context.TryGetValue(transition, out VirtualStateTransition clone)) return clone;
-
-            var cloned = Object.Instantiate(transition);
+            var cloned = Object.Instantiate(transition)!;
             cloned.name = transition.name;
 
             switch (cloned)
@@ -140,8 +144,6 @@ AnimatorTransitionBase ICommitable<AnimatorTransitionBase>.Prepare(CommitContext
 
         void ICommitable<AnimatorTransitionBase>.Commit(CommitContext context, AnimatorTransitionBase obj)
         {
-            _transition = null;
-
             if (DestinationState != null)
             {
                 obj.destinationState = context.CommitObject(DestinationState);
@@ -162,7 +164,6 @@ void ICommitable<AnimatorTransitionBase>.Commit(CommitContext context, AnimatorT
         public void Dispose()
         {
             if (_transition != null) Object.DestroyImmediate(_transition);
-            _transition = null;
         }
 
         protected override IEnumerable<VirtualNode> _EnumerateChildren()