ValidationError when saving completed membership registration

One-sentence description

Current behaviour / Reproducing the bug

Click save while observing a completed registration.

Sentry Issue: CONCREXIT-4

ValueError: 'RegistrationForm' has no field named 'email'.
(14 additional frame(s) were not displayed)
...
  File "django/forms/forms.py", line 180, in errors
    self.full_clean()
  File "django/forms/forms.py", line 383, in full_clean
    self._post_clean()
  File "django/forms/models.py", line 405, in _post_clean
    self._update_errors(e)
  File "django/forms/models.py", line 379, in _update_errors
    self.add_error(None, errors)
  File "django/forms/forms.py", line 353, in add_error
    "'%s' has no field named '%s'." % (self.__class__.__name__, field))

ValidationError: {'email': ['A user with that email address already exists. Login using the existing account and renew the membership by visiting the account settings.'], 'student_number': ['A user with that student number already exists. Login using the existing account and renew the membership by visiting the account settings.'], 'username': ['A user with that username already exists.']}
  File "django/forms/models.py", line 403, in _post_clean
    self.instance.full_clean(exclude=exclude, validate_unique=False)
  File "django/db/models/base.py", line 1152, in full_clean
    raise ValidationError(errors)

Expected behaviour

No crash, maybe even no save button available.