Fix crash when pizza event admin orders pizza

Previous behaviour

When an organizer of a pizza event orders a pizza, the API crashes.

Steps to reproduce:

1. Create an event for which you are organizer.
2. Create an accompanying pizza event.
3. Order pizza using the API/app.
4. Crash.

New behaviour

Ordering works even if you're organizer.

Steps to validate that it works:

1. Create an event for which you are organizer.
2. Create an accompanying pizza event.
3. Order pizza using the API/app.
4. No crash.