windowManager: Check whether metaWindow is NULL after destroy animation

When logging out or terminating gnome-shell, mutter will close 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 #2018 (closed)