When logging out or terminating gnome-shell, mutter will unmanage all
open windows, triggering the window-close animation in gnome-shell and
very quickly after that emitting "kill-window-effects". That means we'll
call _destroyWindowDone() to cleanup our animation data, but at this
point the MetaWindow of the window is already gone, so we get an error
that get_meta_window() returns NULL.
Fix that by checking whether get_meta_window() returned NULL and if it
did, don't access the window.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2018
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1483>
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