def is_compatible(target_type, check_instance):
    """
    Check the target compatibility with the check instance.

    :param target_type: Target subclass, if None, returns True
    :param check_instance: instance of some Check class
    :return: True if the target is None or compatible with the check instance
    """
    if not target_type:
        return True
    return isinstance(check_instance, target_type.get_compatible_check_class())
