I found this while looking in the logs for something else:
End of the error trace is "Argument 6 passed to Flow\Model\WikiReference::__construct() must be an instance of Title, null given".
This sounds similar to T67802: Argument 6 passed to Flow\Model\WikiReference::__construct() must be an instance of Title, null given, but might be a different cause.
mattflaschen@fluorine:/a/mw-log$ zgrep -A 27 d84abaa9 archive/exception.log-20150329*.gz
2015-03-28 18:45:24 mw1246 enwiki: [d84abaa9] /wiki/Special:WhatLinksHere/User:Circeus Flow\Exception\CatchableFatalErrorException from line 35 of /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Model/WikiReference.php: Argument 6 passed to Flow\Model\WikiReference::__construct() must be an instance of Title, null given
#0 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Model/WikiReference.php(35): Flow\RecoverableErrorHandler->__invoke()
#1 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Model/WikiReference.php(63): Flow\Model\WikiReference->__construct()
#2 (): Flow\Model\WikiReference::fromStorageRow()
#3 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Data/Mapper/BasicObjectMapper.php(38): call_user_func()
#4 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Data/ObjectLocator.php(298): Flow\Data\Mapper\BasicObjectMapper->fromStorageRow()
#5 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Data/ObjectManager.php(301): Flow\Data\ObjectLocator->load()
#6 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Data/ObjectLocator.php(105): Flow\Data\ObjectManager->load()
#7 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Data/ObjectLocator.php(55): Flow\Data\ObjectLocator->findMulti()
#8 (): Flow\Data\ObjectLocator->find()
#9 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Data/ManagerGroup.php(129): call_user_func_array()
#10 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/Data/ManagerGroup.php(141): Flow\Data\ManagerGroup->call()
#11 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/ReferenceClarifier.php(112): Flow\Data\ManagerGroup->find()
#12 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/ReferenceClarifier.php(70): Flow\ReferenceClarifier->loadReferencesForPage()
#13 /srv/mediawiki/php-1.25wmf22/extensions/Flow/includes/ReferenceClarifier.php(25): Flow\ReferenceClarifier->getWikiReferences()
#14 /srv/mediawiki/php-1.25wmf22/extensions/Flow/Hooks.php(892): Flow\ReferenceClarifier->getWhatLinksHereProps()
#15 /srv/mediawiki/php-1.25wmf22/includes/Hooks.php(209): FlowHooks::onWhatLinksHereProps()
#16 /srv/mediawiki/php-1.25wmf22/includes/specials/SpecialWhatlinkshere.php(347): Hooks::run()
#17 /srv/mediawiki/php-1.25wmf22/includes/specials/SpecialWhatlinkshere.php(290): SpecialWhatLinksHere->listItem()
#18 /srv/mediawiki/php-1.25wmf22/includes/specials/SpecialWhatlinkshere.php(93): SpecialWhatLinksHere->showIndirectLinks()
#19 /srv/mediawiki/php-1.25wmf22/includes/specialpage/SpecialPage.php(384): SpecialWhatLinksHere->execute()
#20 /srv/mediawiki/php-1.25wmf22/includes/specialpage/SpecialPageFactory.php(581): SpecialPage->run()
#21 /srv/mediawiki/php-1.25wmf22/includes/MediaWiki.php(268): SpecialPageFactory::executePath()
#22 /srv/mediawiki/php-1.25wmf22/includes/MediaWiki.php(571): MediaWiki->performRequest()
#23 /srv/mediawiki/php-1.25wmf22/includes/MediaWiki.php(422): MediaWiki->main()
#24 /srv/mediawiki/php-1.25wmf22/index.php(46): MediaWiki->run()
#25 /srv/mediawiki/w/index.php(3): include()
#26 {main}