fix keeping disposed items in scene. This can cause application to freeze.

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -354,9 +354,12 @@ namespace Avalonia.Rendering
 
                     foreach (var operation in node.DrawOperations)
                     {
-                        _currentDraw = operation;
-                        operation.Item.Render(context);
-                        _currentDraw = null;
+                        if (operation.Item != null)
+                        {
+                            _currentDraw = operation;
+                            operation.Item.Render(context);
+                            _currentDraw = null;
+                        }
                     }
 
                     foreach (var child in node.Children)

src/Avalonia.Visuals/Rendering/SceneGraph/Scene.cs

@@ -150,7 +150,7 @@ namespace Avalonia.Rendering.SceneGraph
 
             index.Add(result.Visual, result);
 
-            foreach (var child in source.Children)
+            foreach (var child in source.Children.Where(x=>!x.Disposed))
             {
                 result.AddChild(Clone((VisualNode)child, result, index));
             }