panel: Ensure already destroyed panels don't receive callbacks
To prevent sporadic shutdown failures:
** Message: 17:08:37.707: Shutting down GNOME Shell
(gnome-shell:91892): Gjs-CRITICAL **: 17:08:37.728: Object .Gjs_ui_panel_Panel (0x5b53524fb030), has been already disposed — impossible to access it. This might be caused by the object having been destroyed from C code using something such as destroy(), dispose(), or remove() vfuncs.
which seem more likely to happen with triple buffering (mutter!1441 (comment 2323261)).