code: def get_newsitem_model(model_string):
      
      try:
          NewsItem = apps.get_model(model_string)
          assert issubclass(NewsItem, AbstractNewsItem)
      except (ValueError, LookupError, AssertionError):
          raise ValueError('Invalid news item model string'.format(model_string))
      return NewsItem
docs: Get the NewsItem model from a model string. Raises ValueError if the model
      string is invalid, or references a model that is not a NewsItem.