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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_newsitem_model(model_string):
 """ 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. """

    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