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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 _check_id_validity(self, p_ids):
 """ Checks if there are any invalid todo IDs in p_ids list. Returns proper error message if any ID is invalid and None otherwise. """

        errors = []
        valid_ids = self.todolist.ids()

        if len(p_ids) == 0:
            errors.append('No todo item was selected')
        else:
            errors = ["Invalid todo ID: {}".format(todo_id)
                      for todo_id in p_ids - valid_ids]

        errors = '\n'.join(errors) if errors else None
        return errors