docs: def notification_model(self):
          """Returns the Notification 'model' instance associated
          with this notification.
          """
          NotificationModel = django_apps.get_model("edc_notification.notification")
          # trigger exception if this class is not registered.
          site_notifications.get(self.name)
          try:
              notification_model = NotificationModel.objects.get(name=self.name)
          except ObjectDoesNotExist:
              site_notifications.update_notification_list()
              notification_model = NotificationModel.objects.get(name=self.name)
          return notification_model