menutrackeritem: protect against use-after-free

menutrackeritem: protect against use-after-free

With recent updates to GLib, I now see cases where we can hit a state that
has finalized before notify (which will bump the ref count back up). This
is evident in GNOME Text Editor when showing a language submenu from a
popover, and then dismissing the popover and subsequently the tab.

With the previous commit, we at least get a warning like this, which helped
track down the issue.

 Gtk-CRITICAL **: gtk_action_observable_unregister_observer: assertion 'GTK_IS_ACTION_OBSERVABLE (observable)' failed
 GLib-GObject-CRITICAL **: g_object_ref: assertion '!object_already_finalized' failed

This patch fixes both of those criticals.

Fixes #5009