Validate the message.

        Ensure all the required fields are present and not None.

def validate(self):
        """
        Validate the message.

        Ensure all the required fields are present and not None.
        """
        required_fields = list(filter(lambda x: self._fields[x], self._fields.keys()))
        for field in (self._top_fields + required_fields):
            if not hasattr(self, field) or hasattr(self, field) and getattr(self, field) == None:
                raise MissingFieldError(field)