def write_null_portable(self, field_name, factory_id, class_id):
raise NotImplementedError()<EOL>
To write a null portable value, user needs to provide class and factoryIds of related class.

:param field_name: (str), name of the field.
:param factory_id: (int), factory id of related portable class.
:param class_id: (int), class id of related portable class.