Don't return detached controls in HitTest.

If a control has been detached from the visual tree but the scene hasn't yet been updated, don't return the control as part of a hit-test.

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

@@ -173,7 +173,7 @@ namespace Avalonia.Rendering.SceneGraph
                         }
                     }
 
-                    if (node.HitTest(p))
+                    if (node.HitTest(p) && node.Visual.IsAttachedToVisualTree)
                     {
                         yield return node.Visual;
                     }