code: def handle_error(self, error, data):
          
          required_messages = {'Missing data for required field.',
                               'Field may not be null.'}
          for field_name in error.field_names:
              for i, msg in enumerate(error.messages[field_name]):
                  if isinstance(msg, _LazyString):
                      msg = str(msg)
                  if msg in required_messages:
                      label = title_case(field_name)
                      error.messages[field_name][i] = f'{label} is required.'
docs: Customize the error messages for required/not-null validators with
          dynamically generated field names. This is definitely a little hacky (it
          mutates state, uses hardcoded strings), but unsure how better to do it