Prevent NRE in VisualNode.HitTest.

Not sure how this is happening but judging by #2758, it can happen. Defensively check for a null `Item` to prevent this.

Fixes #2758.

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

@@ -236,7 +236,7 @@ namespace Avalonia.Rendering.SceneGraph
         {
             foreach (var operation in DrawOperations)
             {
-                if (operation.Item.HitTest(p))
+                if (operation?.Item?.HitTest(p) == true)
                 {
                     return true;
                 }