IGitt, _repository access: TypeError: must be str, not NoneType

https://sentry.io/gitmate-ug-haftungsbeschrankt/gitmateio/issues/396521082/

TypeError: must be str, not NoneType
  File "celery/app/trace.py", line 367, in trace_task
    R = retval = fun(*args, **kwargs)
  File "celery/app/trace.py", line 622, in __protected_call__
    return self.run(*args, **kwargs)
  File "plugins/gitmate_issue_stale_reminder/responders.py", line 27, in add_stale_label_to_issues
    for issue in repo.search_issues(updated_before=minimum_issue_update_time):
  File "IGitt/GitHub/GitHubRepository.py", line 504, in search_issues
    updated_before):
  File "IGitt/GitHub/GitHubRepository.py", line 452, in _search
    query = ' type:' + issue_type + ' state:open repo:' + self._repository

TypeError: must be str, not NoneType

I guess self._repository is None because issue_type isn't.