```
// If a notification is never used, its type will not be added to the database
// nor its id cached. If this method is called by an extension during the
// purge step, and that extension never used its notifications, 
// get_notification_type_id() will throw an exception. However,
// because no notification type was added to the database,
// there is nothing to delete, so we can silently drop the exception.
```