Prevent Shift Drag Crash when single group selected

src/DynamoCoreWpf/Commands/DynamoCommands.cs

@@ -112,9 +112,20 @@ void OnModelCommandCompleted(DynamoModel.RecordableCommand command)
                     var dragC = command as DynamoModel.DragSelectionCommand;
 
                     if (DynamoModel.DragSelectionCommand.Operation.BeginDrag == dragC.DragOperation)
-                        CurrentSpaceViewModel.BeginDragSelection(dragC.MouseCursor);
+                    {
+                        try
+                        {
+                            CurrentSpaceViewModel.BeginDragSelection(dragC.MouseCursor);
+                        }
+                        catch (Exception ex)
+                        {
+                            model.Logger.Log(ex.Message);
+                        }
+                    }
                     else
+                    {
                         CurrentSpaceViewModel.EndDragSelection(dragC.MouseCursor);
+                    }
                     break;
 
                 case "DeleteModelCommand":

src/DynamoCoreWpf/Properties/Resources.en-US.resx

@@ -3252,4 +3252,7 @@ You can manage this in Preferences -&gt; Security.</value>
   <data name="UnableToAccessTrustedDirectory" xml:space="preserve">
     <value>Unable To Access Trusted Directory</value>
   </data>
+  <data name="InvalidDraggingOperationMessgae" xml:space="preserve">
+    <value>Nothing is being dragged. If you see this message, most likely your recent Dynamo interaction is not recommended.</value>
+  </data>
 </root>

src/DynamoCoreWpf/Properties/Resources.resx

@@ -3239,4 +3239,7 @@ You can manage this in Preferences -&gt; Security.</value>
   <data name="UnableToAccessTrustedDirectory" xml:space="preserve">
     <value>Unable To Access Directory</value>
   </data>
+  <data name="InvalidDraggingOperationMessgae" xml:space="preserve">
+    <value>Nothing is being dragged. If you see this message, most likely your recent Dynamo interaction is not recommended.</value>
+  </data>
 </root>

src/DynamoCoreWpf/ViewModels/Core/StateMachine.cs

@@ -27,7 +27,7 @@ partial class WorkspaceViewModel
 
         #region State Machine Related Methods/Data Members
 
-        private StateMachine stateMachine = null;
+        private readonly StateMachine stateMachine = null;
         private List<DraggedNode> draggedNodes = new List<DraggedNode>();
 
         // When a new connector is created or a single connector is selected,
@@ -146,19 +146,12 @@ internal void BeginDragSelection(Point2D mouseCursor)
 
             if (draggedNodes.Count <= 0) // There is nothing to drag.
             {
-                string message = "Shouldn't get here if nothing is dragged";
-                throw new InvalidOperationException(message);
+                throw new InvalidOperationException(Wpf.Properties.Resources.InvalidDraggingOperationMessgae);
             }
         }
 
         internal void UpdateDraggedSelection(Point2D mouseCursor)
         {
-            if (draggedNodes.Count <= 0)
-            {
-                throw new InvalidOperationException(
-                    "UpdateDraggedSelection cannot be called now");
-            }
-
             foreach (DraggedNode draggedNode in draggedNodes)
                 draggedNode.Update(mouseCursor);
         }
@@ -343,15 +336,15 @@ private void RecordSelectionForUndo(Point2D mousePoint)
             List<ModelBase> changedPositionModels = new List<ModelBase>();
             foreach (DraggedNode draggedNode in draggedNodes)
             {
-                //Checks if the draggednoded has changed its position
+                //Checks if the dragged node has changed its position
                 if (draggedNode.HasChangedPosition(mousePoint))
                 {
                     ModelBase model = DynamoSelection.Instance.Selection.
                         Where((x) => (x is ModelBase)).Cast<ModelBase>().FirstOrDefault(x => x.GUID == draggedNode.guid);
 
                     changedPositionModels.Add(model);
 
-                    //The nodes are being reseted to inital position for recording the model purposes 
+                    //The nodes are being reseted to initial position for recording the model purposes 
                     draggedNode.UpdateInitialPosition();
                 }
             }
@@ -363,15 +356,13 @@ private void RecordSelectionForUndo(Point2D mousePoint)
         private void OnDragSelectionStarted(object sender, EventArgs e)
         {
             //Debug.WriteLine("Drag started : Visualization paused.");
-            if (DragSelectionStarted != null)
-                DragSelectionStarted(sender, e);
+            DragSelectionStarted?.Invoke(sender, e);
         }
 
         private void OnDragSelectionEnded(object sender, EventArgs e)
         {
             //Debug.WriteLine("Drag ended : Visualization unpaused.");
-            if (DragSelectionEnded != null)
-                DragSelectionEnded(sender, e);
+            DragSelectionEnded?.Invoke(sender, e);
         }
 
         #endregion
@@ -384,10 +375,12 @@ private void OnDragSelectionEnded(object sender, EventArgs e)
         /// </summary>
         public class DraggedNode
         {
-            double deltaX = 0, deltaY = 0;
-            ILocatable locatable = null;
+            private readonly double deltaX = 0;
+            private readonly double deltaY = 0;
+            readonly ILocatable locatable = null;
             internal Guid guid;
-            double initialPositionX = 0, initialPositionY = 0;
+            private readonly double initialPositionX = 0;
+            private readonly double initialPositionY = 0;
 
             /// <summary>
             /// Construct a DraggedNode for a given ILocatable object.
@@ -399,9 +392,6 @@ public class DraggedNode
             /// <param name="mouseCursor">The mouse cursor at the point this 
             /// DraggedNode object is constructed. This is used to determine the 
             /// offset of the ILocatable from the mouse cursor.</param>
-            /// <param name="region">The region within which the ILocatable can 
-            /// be moved. However, the movement of ILocatable will be limited by 
-            /// region and that it cannot be moved beyond the region.</param>
             /// 
             public DraggedNode(ILocatable locatable, Point2D mouseCursor)
             {