<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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)