Member can be accepted and have paid, but status is not completed

This ValueError happens sometimes while processing payments for new members or renewals. I'm not quite sure how to reproduce it, but sometimes when setting a registration or renewal to paid. The status of the renewal or registration will afterwards be "Accepted" instead of "Completed", and a 500 error is shown.

Sentry Issue: CONCREXIT-H

ValueError: The given username must be set
(19 additional frame(s) were not displayed)
...
  File "registrations/signals.py", line 12, in post_payment_save
    services.process_payment(instance)
  File "registrations/services.py", line 414, in process_payment
    member = _create_member_from_registration(registration)
  File "registrations/services.py", line 286, in _create_member_from_registration
    last_name=registration.last_name
  File "django/contrib/auth/models.py", line 151, in create_user
    return self._create_user(username, email, password, **extra_fields)
  File "django/contrib/auth/models.py", line 140, in _create_user
    raise ValueError('The given username must be set')