def handle_error(self, error, data):
        """
        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
        """
        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.'
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