window: Warn if try to focus unmanaging windows

https://gitlab.gnome.org/GNOME/mutter/merge_requests/307

src/core/window.c

@@ -8518,6 +8518,8 @@ meta_window_shortcuts_inhibited (MetaWindow         *window,
 gboolean
 meta_window_is_focusable (MetaWindow *window)
 {
+  g_return_val_if_fail (!window->unmanaging, FALSE);
+
   return META_WINDOW_GET_CLASS (window)->is_focusable (window);
 }