Add: NullRef when adding graph with shadow keys

[bricelam]

class TestAssembly
{
    [Key]
    public string Name { get; set; }
    public ICollection<TestClass> Classes { get; } = new List<TestClass>();
}

class TestClass
{
    public TestAssembly Assembly { get; set; }
    public string Name { get; set; }
    public ICollection<Test> Tests { get; } = new List<Test>();
}

class Test
{
    public TestClass Class { get; set; }
    public string Name { get; set; }
}

class Context : DbContext
{
    public DbSet<TestAssembly> Assemblies { get; set; }
    public DbSet<TestClass> Classes { get; set; }
    public DbSet<Test> Tests { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
        => options.UseSqlite("Data Source=ConsoleApplication1.db");

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<TestClass>(
            x =>
            {
                x.Property<string>("AssemblyName");
                x.HasKey("AssemblyName", nameof(TestClass.Name));
                x.HasOne(c => c.Assembly).WithMany(a => a.Classes)
                    .HasForeignKey("AssemblyName");
            });
        modelBuilder.Entity<Test>(
            x =>
            {
                x.Property<string>("AssemblyName");
                x.HasKey("AssemblyName", "ClassName", nameof(Test.Name));
                x.HasOne(t => t.Class).WithMany(c => c.Tests)
                    .HasForeignKey("AssemblyName", "ClassName");
            });
    }
}

class Program
{
    static void Main()
    {
        using (var db = new Context())
        {
            db.Database.EnsureDeleted();
            db.Database.EnsureCreated();

            var assembly = new TestAssembly { Name = "Assembly1" };
            var testClass = new TestClass { Assembly = assembly, Name = "Class1" };
            db.Tests.Add(new Test { Class = testClass, Name = "Test1" });

            // Throws NullReferenceException
            db.SaveChanges();
        }
    }
}

System.NullReferenceException: Object reference not set to an instance of an object.
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.CompositePrincipalKeyValueFactory.CompositeComparer.Equals(Object[] x, Object[] y)
    at System.Collections.Generic.Dictionary`2.Remove(TKey key)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IdentityMap`1.Remove(TKey key, InternalEntityEntry entry)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NullableKeyIdentityMap`1.RemoveUsingRelationshipSnapshot(InternalEntityEntry entry)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.UpdateIdentityMap(InternalEntityEntry entry, IKey key)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.KeyPropertyChanged(InternalEntityEntry entry, IProperty property, IReadOnlyList`1 containingPrincipalKeys, IReadOnlyList`1 containingForeignKeys, Object oldValue, Object newValue)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.<>c__DisplayClass9_0.<KeyPropertyChanged>b__0(IKeyListener l)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.Dispatch(Action`1 action)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.KeyPropertyChanged(InternalEntityEntry entry, IProperty property, IReadOnlyList`1 keys, IReadOnlyList`1 foreignKeys, Object oldValue, Object newValue)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.DetectKeyChange(InternalEntityEntry entry, IProperty property)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.PropertyChanged(InternalEntityEntry entry, IPropertyBase propertyBase, Boolean setModified)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.<>c__DisplayClass10_0.<PropertyChanged>b__0(IPropertyListener l)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.Dispatch(Action`1 action)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.PropertyChanged(InternalEntityEntry entry, IPropertyBase property, Boolean setModified)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetProperty(IPropertyBase propertyBase, Object value, Boolean setModified)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.SetForeignKeyProperties(InternalEntityEntry dependentEntry, InternalEntityEntry principalEntry, IForeignKey foreignKey, Boolean setModified)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.FixupToPrincipal(InternalEntityEntry dependentEntry, InternalEntityEntry principalEntry, IForeignKey foreignKey, Boolean setModified)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.DelayedFixup(InternalEntityEntry entry, INavigation navigation, InternalEntityEntry referencedEntry)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.InitialFixup(InternalEntityEntry entry, Boolean fromQuery)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.StateChanged(InternalEntityEntry entry, EntityState oldState, Boolean skipInitialFixup, Boolean fromQuery)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.<>c__DisplayClass6_0.<StateChanged>b__0(IEntityStateListener l)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.Dispatch(Action`1 action)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.StateChanged(InternalEntityEntry entry, EntityState oldState, Boolean skipInitialFixup, Boolean fromQuery)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState oldState, EntityState newState, Boolean acceptChanges)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState entityState, Boolean acceptChanges)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.PaintAction(EntityEntryGraphNode node)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph(EntityEntryGraphNode node, Func`2 handleNode)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph(EntityEntryGraphNode node, Func`2 handleNode)
    at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.AttachGraph(InternalEntityEntry rootEntry, EntityState entityState)
    at Microsoft.EntityFrameworkCore.DbContext.SetEntityState(InternalEntityEntry entry, EntityState entityState)
    at Microsoft.EntityFrameworkCore.DbContext.SetEntityState[TEntity](TEntity entity, EntityState entityState)
    at Microsoft.EntityFrameworkCore.DbContext.Add[TEntity](TEntity entity)
    at Microsoft.EntityFrameworkCore.Internal.InternalDbSet`1.Add(TEntity entity)
    at ConsoleApplication1.Program.Main()
    at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
    at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
    at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
    at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
    at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
    at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
    at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
    at System.Threading.ThreadHelper.ThreadStart()