windowManager: Check whether metaWindow is NULL after destroy animation
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.