messageTray: Avoid notification double destruction after activation

messageTray: Avoid notification double destruction after activation

The activation handler of a notification may indirectly have already
destroyed it before `activate()` continues and tries to destroy it.

This happened for example in the `FdoNotificationDaemon` activation
handler when no default actions are specified, which ends up calling
`destroyNonResidentNotifications()`.

Another case where this could happen is the `WindowAttentionHandler`
which activates the window and upon activation of the window destroys
the entire notification source, which also destroys the notification.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5421
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4113>
(cherry picked from commit 2fe0c2b4b23e526801ade3d54dedb578af04307c)