Cannot register payment of external event registration

One-sentence description

I cannot register a payment of someone that is manually registered for an event, but is not a member of Thalia.

Current behaviour / Reproducing the bug

1. Open an event
2. Manually add an registration - not a member but fill in a empty name
3. Save
4. In the overview, change the payment from 'not paid' to 'pin' or 'cash'
5. Reload the page, the payment is not registered
6. profit

Expected behaviour

Should be clear I guess