Merge pull request #7373 from AvaloniaUI/fix-nre-menu-close

Fix NRE on window close with opened menu

src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs

@@ -101,7 +101,7 @@ public virtual void Detach(IMenu menu)
                 root.Deactivated -= WindowDeactivated;
             }
 
-            if (_root is TopLevel tl)
+            if (_root is TopLevel tl && tl.PlatformImpl != null)
                 tl.PlatformImpl.LostFocus -= TopLevelLostPlatformFocus;
 
             _inputManagerSubscription?.Dispose();