Yea, that's a bit of a problem...

The reason for the error is that the boardrules services are not loaded since the extension is disabled, so of course it wouldn't find the notification type in the available services.

For now, remove the try/catch here, add a todo note at the top noting this issue, and before the switch just return parent::purge_step(); Then we don't have to hold this back for that phpBB bug.

If you could open a topic on area51 to discuss this issue and a bug ticket that would be great!