messageTray: Ensure _updateDatetimeId idle callback is cancelled

If the idle callback is not explicitly removed when the notification is
destroyed, it may be invoked after destruction, and try to modify the
notification after it’s been freed.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3352>

js/ui/messageTray.js

@@ -475,6 +475,11 @@ export class Notification extends GObject.Object {
 
     destroy(reason = NotificationDestroyedReason.DISMISSED) {
         this.emit('destroy', reason);
+
+        if (this._updateDatetimeId)
+            GLib.source_remove(this._updateDatetimeId);
+        delete this._updateDatetimeId;
+
         this.run_dispose();
     }
 }