PhotoCalTask passes a config object as the first argument to the DirectMatchTask constructor, however DirectMatchTask defines butler as its first argument, not config. This causes the config object to be assigned to the butler argument. If the referenceObjectLoader is passed to DirectMatchTask's constructor then this has no impact. However if the referenceObjectLoader is None this will cause the task to fail.

See:
https://github.com/lsst/pipe_tasks/blob/master/python/lsst/pipe/tasks/photoCal.py#L256
and:
https://github.com/lsst/meas_astrom/blob/master/python/lsst/meas/astrom/directMatch.py#L42