If something (i.e. gnome-shell or an extension) tries to activate an unmanaged
window, we should warn about this and avoid to perform further actions as this
could lead to a crash of mutter, since the window has not valid flags (like
workspace) set anymore at this stage.
Fixes https://gitlab.gnome.org/GNOME/mutter/issues/580https://gitlab.gnome.org/GNOME/mutter/merge_requests/564
(cherry picked from commit a6fc656e917fd48b8708b8d9f4bf9f8b15581313)