shell/app: Warn instead of crashing if disposed before startup notification

shell/app: Warn instead of crashing if disposed before startup notification

This is currently easy to reproduce by launching and immediately closing
Chrome, then closing the shell. But automated crash reports show it can
also happen while using the shell, during a garbage collection run.

It's currently the top gnome-shell crasher in Ubuntu, and mostly with
Chromium/Electron apps. But regardless of the offender, we shouldn't
ever trust application behaviour to keep the shell alive.

Bug: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7045
Bug-Ubuntu: https://bugs.launchpad.net/bugs/2037055
Forwarded: https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3457

Gbp-Pq: Name shell-app-Warn-instead-of-crashing-if-disposed-before-sta.patch