Validates arguments.

def validate_args(self):
        """
        Validates arguments.
        """
        from ..mixins import ModelMixin

        for arg in ("instance", "decider", "identifier", "fields", "default_language"):
            if getattr(self, arg) is None:
                raise AttributeError("%s must not be None" % arg)

        if not isinstance(self.instance, (ModelMixin,)):
            raise ImproperlyConfigured('"instance" argument must be a Linguist model')

        if not issubclass(self.decider, (models.Model,)):
            raise ImproperlyConfigured(
                '"decider" argument must be a valid Django model'
            )