code: def get_referenced_user(self):
          
  
          if self._user_person is not None:
              return self._user_person
  
          if self._user_company is not None:
              return self._user_company
  
          if self._user_api_key is not None:
              return self._user_api_key
  
          raise BunqException(self._ERROR_ALL_FIELD_IS_NULL)
docs: :rtype: BunqModel