52fdd24467
To check if a subsurface is effectively synchronized, we walk the subsurface hierarchy to look for a non-subsurface parent or a subsurface being synchronized. However, when client is closing, the parent surface might already be gone, in which case we end up with a surface being NULL which causes a NULL pointer dereference and a crash. Check if the parent surface is NULL to avoid the crash, and consider it's already synchronized if it is NULL to avoid further updates. Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/124