58cc091134
A window can be unmanaged without asking the client to do it, for example as a side effect of a parent window being unmanaged, if the child window was a attached dialog. This means that the client might still make requests post updates to it after that it was unmapped. Handle this gracefully by NULL-checking the surface's MetaWindow pointer. We're not loosing any state due to this, as if the client wants to map the same surface again, it needs to either reassign it the toplevel role, or reset the xdg-toplevel, both resulting in all state being lost anyway. https://gitlab.gnome.org/GNOME/mutter/issues/240 (cherry picked from commit 5fd0f62a62a194ffd8e64d177f389912a582f8e1)