Comment 7 for bug 1751338

None type appears to be by design:

https://github.com/openstack/charm-designate-bind/blob/master/src/lib/charm/openstack/designate_bind.py#L86

def get_sync_time():
    """Use the singleton from the DesignateBindCharm to retrieve the time of
    the published zone zync target
    :returns: str or None. Current sync target creation time if available, None
                           if not.
    """
    return DesignateBindCharm.singleton.get_sync_time()