Dispose the grid hack in the correct place.

The grid hack was initiated when attached to the visual tree but the dispose had been placed in `OnDetachedFromLogicalTree` by mistake. Place it in `OnDetachedFromVisualTree`.

src/Avalonia.Controls/MenuItem.cs

@@ -323,9 +323,6 @@ namespace Avalonia.Controls
             {
                 Command.CanExecuteChanged -= CanExecuteChanged;
             }
-
-            _gridHack?.Dispose();
-            _gridHack = null;
         }
 
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
@@ -351,6 +348,14 @@ namespace Avalonia.Controls
             }
         }
 
+        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnDetachedFromVisualTree(e);
+
+            _gridHack?.Dispose();
+            _gridHack = null;
+        }
+
         /// <summary>
         /// Called when the <see cref="MenuItem"/> is clicked.
         /// </summary>