Fix Catchable fatal error: Object of class WFReply could not be converted to string

Patch from
https://www.mediawiki.org/w/index.php?title=Extension_talk:WikiForum&curid=59523&diff=1160684&oldid=1152444
by MediaWiki.org user 31.221.83.26

Change-Id: I6ca96cd379ca85d1d4692e617d86acfb9b638290