shell/app: Warn instead of crashing if disposed before stopped

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. So that's probably why it's also the top gnome-shell crasher in Ubuntu.

Regardless of the cause, there's no need to dump core when a warning will suffice.

Closes: #7045
Related: https://bugs.launchpad.net/bugs/2037055