<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_concrete_model(model):
 """ Get model defined in Meta. :param str or django.db.models.Model model: :return: model or None :rtype django.db.models.Model or None: :raise ValueError: model is not found or abstract """

    if not(inspect.isclass(model) and issubclass(model, models.Model)):
        model = get_model_by_name(model)

    return model