We could move this back to the rest of the kwarg setting area, if we instead check the type of the model field.

    if issubclass(model_field, (models.CharField, models.TextField)):
        if model_field.null:
            kwargs['allow_none'] = True

We could then leave the `return self.field_mapping[...(**kwargs)` line as it is.
Would be a little easier to follow the code then.