def convert_value_to_list(dictionary, key):
    """
    Given a dictionary and a key, make sure the key value is a list type.
    If the key does not exist in the dictionary, just return None.
    :param dictionary: A dictionary instance.
    :param key: A dictionary key.
    :return: Return a list type.
    """
    try:
        value = dictionary.get(key)
        return None if value is None else value if type(value) is list else [value]
    except:
        raise ValueError("The value associated with '{}' is expected to be a list type - {}.".format(key, dictionary))