Moving a Control from one parent to another throws an Exception

[RationalFragile]

Describe the bug
If you have one control, you move it from parent1 to parent2, a NullReferenceException is thrown:

Exception details

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Avalonia.Visuals
  StackTrace:
   at Avalonia.Rendering.DeferredRenderer.Render(IDrawingContextImpl context, VisualNode node, IVisual layer, Rect clipBounds) in C:\Avalonia\src\Avalonia.Visuals\Rendering\DeferredRenderer.cs:line 433

  This exception was originally thrown at this call stack:
    Avalonia.Rendering.DeferredRenderer.Render(Avalonia.Platform.IDrawingContextImpl, Avalonia.Rendering.SceneGraph.VisualNode, Avalonia.VisualTree.IVisual, Avalonia.Rect) in DeferredRenderer.cs

To Reproduce

<DockPanel>
    <Button Command="{Binding Move}">Move</Button>
    <Grid Name="Parent">
        <TextBlock>content</TextBlock>
    </Grid>
    <Grid Name="Parent2">

    </Grid>
</DockPanel>

And

public void Move(){
  var parent = this.FindControl<Grid>("Parent");
  var parent2 = this.FindControl<Grid>("Parent2");
  if (parent.Children.Count > 0)
  {
      System.Console.WriteLine("1 -> 2");
      var child = parent.Children[0];
      parent.Children.Clear();
      parent2.Children.Add(child);
  }
  else
  {
      System.Console.WriteLine("1 <- 2");
      var child = parent2.Children[0];
      parent2.Children.Clear();
      parent.Children.Add(child);
  }
}

Expected behavior
TextBlock should be removed from the old parent and added to the new parent.

Desktop (please complete the following information):

• OS: Windows 10 (20H2 19042.868)
• Version 0.10.11

Additional context
It doesn't work with other types of parents as well (e.g. Border).
Also, the Exception is thrown here:

private void Avalonia.Visuals.Rendering.DeferredRenderer.Render
(IDrawingContextImpl context, VisualNode node, IVisual? layer, Rect clipBounds)
        {
[...]
                    for (int i = 0; i < drawOperationsCount; i++)
                    {
                        var operation = drawOperations[i];
                        _currentDraw = operation;
                        operation.Item.Render(context); // << Item is null
                        _currentDraw = null;
                    }
[...]
}

Simply adding a null check at operation.Item?.Render(context); will cause other Exceptions:

Exception details

System.ObjectDisposedException
  HResult=0x80131622
  Message=Cannot access a disposed object.
Object name: 'Ref<System.IDisposable>'.
  Source=Avalonia.Base
  StackTrace:
   at Avalonia.Utilities.RefCountable.Ref`1.Clone() in C:\Avalonia\src\Avalonia.Base\Utilities\Ref.cs:line 186
   at Avalonia.Rendering.SceneGraph.VisualNode.Clone(IVisualNode parent) in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\VisualNode.cs:line 262
   at Avalonia.Rendering.SceneGraph.Scene.Clone(VisualNode source, IVisualNode parent, Dictionary`2 index) in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\Scene.cs:line 165
   at Avalonia.Rendering.SceneGraph.Scene.Clone(VisualNode source, IVisualNode parent, Dictionary`2 index) in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\Scene.cs:line 179
   at Avalonia.Rendering.SceneGraph.Scene.Clone(VisualNode source, IVisualNode parent, Dictionary`2 index) in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\Scene.cs:line 179
   at Avalonia.Rendering.SceneGraph.Scene.Clone(VisualNode source, IVisualNode parent, Dictionary`2 index) in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\Scene.cs:line 179
   at Avalonia.Rendering.SceneGraph.Scene.Clone(VisualNode source, IVisualNode parent, Dictionary`2 index) in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\Scene.cs:line 179
   at Avalonia.Rendering.SceneGraph.Scene.Clone(VisualNode source, IVisualNode parent, Dictionary`2 index) in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\Scene.cs:line 179
   at Avalonia.Rendering.SceneGraph.Scene.Clone(VisualNode source, IVisualNode parent, Dictionary`2 index) in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\Scene.cs:line 179
   at Avalonia.Rendering.SceneGraph.Scene.CloneScene() in C:\Avalonia\src\Avalonia.Visuals\Rendering\SceneGraph\Scene.cs:line 91
   at Avalonia.Rendering.DeferredRenderer.UpdateScene() in C:\Avalonia\src\Avalonia.Visuals\Rendering\DeferredRenderer.cs:line 635
   at Avalonia.Rendering.DeferredRenderer.UpdateSceneIfNeeded() in C:\Avalonia\src\Avalonia.Visuals\Rendering\DeferredRenderer.cs:line 619
   at Avalonia.Threading.JobRunner.Job.Avalonia.Threading.JobRunner.IJob.Run() in C:\Avalonia\src\Avalonia.Base\Threading\JobRunner.cs:line 177
   at Avalonia.Threading.JobRunner.RunJobs(Nullable`1 priority) in C:\Avalonia\src\Avalonia.Base\Threading\JobRunner.cs:line 37
   at Avalonia.Win32.Win32Platform.WndProc(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam) in C:\Avalonia\src\Windows\Avalonia.Win32\Win32Platform.cs:line 263
   at Avalonia.Win32.Interop.UnmanagedMethods.DispatchMessage(MSG& lpmsg)
   at Avalonia.Win32.Win32Platform.RunLoop(CancellationToken cancellationToken) in C:\Avalonia\src\Windows\Avalonia.Win32\Win32Platform.cs:line 210
   at Avalonia.Threading.Dispatcher.MainLoop(CancellationToken cancellationToken) in C:\Avalonia\src\Avalonia.Base\Threading\Dispatcher.cs:line 61
   at Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.Start(String[] args) in C:\Avalonia\src\Avalonia.Controls\ApplicationLifetimes\ClassicDesktopStyleApplicationLifetime.cs:line 132
   at Avalonia.ClassicDesktopStyleApplicationLifetimeExtensions.StartWithClassicDesktopLifetime[T](T builder, String[] args, ShutdownMode shutdownMode) in C:\Avalonia\src\Avalonia.Controls\ApplicationLifetimes\ClassicDesktopStyleApplicationLifetime.cs:line 187
   at Sandbox.Program.Main(String[] args) in C:\Avalonia\samples\Sandbox\Program.cs:line 7

[RationalFragile]

The issue only happens when you swap the children of two parents and one of them has no children.

Here is an updated test code:

<DockPanel>
  <Button Command="{Binding $parent[Window].Move}">Move</Button>
  <Grid Name="Parent">
    <TextBlock>content</TextBlock>
  </Grid>
  <Grid Name="Parent2">
<!-- if you uncomment this child, the code will execute without exceptions-->
    <!-- <TextBlock>content2</TextBlock> -->
  </Grid>
</DockPanel>

public void Move() {
  var parent = this.FindControl<Grid>("Parent");
  var parent2 = this.FindControl<Grid>("Parent2");
  List<IControl> c1 = new();
  foreach (var c in parent.Children) {
      c1.Add(c);
  }

  List<IControl> c2 = new();
  foreach (var c in parent2.Children) {
      c2.Add(c);
  }
  parent.Children.Clear();
  parent2.Children.Clear();

  parent.Children.AddRange(c2);
  parent2.Children.AddRange(c1);
}

(Also, same behavior for Border.Child, if one of the parents has no child, it will throw...)
The issue still exists in 0.10.12

[RationalFragile]

Following my last comment, a temporary workaround is to simply check if the parent has no children/child and assign to it an empty control before assigning the swapped child. (And maybe delete it later depending on your logic...) Like this:

            var parent = this.FindControl<Grid>("Parent");
            var parent2 = this.FindControl<Grid>("Parent2");

           // avoiding assigning a swapped child to a parent that doesn't have children
            if (parent.Children.Count == 0) {
                parent.Children.Add(new Control());
            }
            if (parent2.Children.Count == 0) {
                parent2.Children.Add(new Control());
            }

            List<IControl> c1 = new();
            foreach (var c in parent.Children) {
                c1.Add(c);
            }

            List<IControl> c2 = new();
            foreach (var c in parent2.Children) {
                c2.Add(c);
            }
            parent.Children.Clear();
            parent2.Children.Clear();

            parent.Children.AddRange(c2);
            parent2.Children.AddRange(c1);