When exiting Boxes while a notification is shown (I tried
with the oVirt authentication popup), Boxes segfaults in
NotificationBar::cancel().
NotificationBar::cancel() is iterating over the list of active
notifications and calling gtk_widget_destroy() on them.

GdNotification::destroy() is emitting the GdNotification::discarded
signal, which NotificationBar catches to remove the current
notification from the list of active notifications.

After calling gtk_widget_destroy(), NotificationBar::cancel() tries
to remove the current notification from the active notifications,
which is useless as this was just done in the callback for
NotificationBar::dismissed. Even worse, this caused a crash in my
testing as active_notifications only contained 1 item, so it was empty
after destroying the widget, and
active_notifications.remove (active_notifications.data); would attempt
to dereference a NULL pointer, causing a crash.