Previously, it was treated as an invariant that whenever
app->running_state was non-NULL, app->running_state->windows was also
non-NULL.

However, on #750 and #822, we see that the backtraces for GNOME Shell
3.30.x contain _shell_app_remove_window() line 1110. In 3.30.x,
that line is at a point in _shell_app_remove_window() where the
app->running_state->windows invariant does not hold: window has already
been removed from app->running_state->windows, but app->running_state
has not yet been cleared. (The same applies in 3.32.x, but with
different line numbers.)

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: GNOME/gnome-shell#750
Closes: GNOME/gnome-shell#822