MediaWiki gives error, when try to use preview of deleted flow pages.
See:
{F2473474}
{F2473476}
{F2473477} "Spezial:Wiederherstellen" is german, the english name for this page is "Special:Undelete"
For those, who has sysop access to de.wikipedia.beta.wmflabs.org, the page im undeleting there is http://de.wikipedia.beta.wmflabs.org/wiki/Benutzer_Diskussion:Luke081515
---
We need to implement BoardContentHandler->createDifferenceEngine. I haven't found a way yet to opt out of diffs being available, so in the meantime we can return some representation, e.g. the JSON or a placeholder.
Message is:
```
[5bacd45b745a53a372e4150c] /w/index.php?title=Scen_c_4&diff=4724&oldid=4723 MWException from line 804 of /vagrant/mediawiki/includes/diff/DifferenceEngine.php: Diff not implemented for Flow\Content\BoardContent; override generateContentDiffBody to fix this.
Backtrace:
#0 /vagrant/mediawiki/includes/diff/DifferenceEngine.php(747): DifferenceEngine->generateContentDiffBody(Flow\Content\BoardContent, WikitextContent)
#1 /vagrant/mediawiki/includes/diff/DifferenceEngine.php(681): DifferenceEngine->getDiffBody()
#2 /vagrant/mediawiki/includes/diff/DifferenceEngine.php(651): DifferenceEngine->getDiff(string, string, string)
#3 /vagrant/mediawiki/includes/diff/DifferenceEngine.php(454): DifferenceEngine->showDiff(string, string, string)
#4 /vagrant/mediawiki/includes/page/Article.php(787): DifferenceEngine->showDiffPage(boolean)
#5 /vagrant/mediawiki/includes/page/Article.php(498): Article->showDiffPage()
#6 /vagrant/mediawiki/includes/actions/ViewAction.php(44): Article->view()
#7 /vagrant/mediawiki/includes/MediaWiki.php(504): ViewAction->show()
#8 /vagrant/mediawiki/includes/MediaWiki.php(288): MediaWiki->performAction(Article, Title)
#9 /vagrant/mediawiki/includes/MediaWiki.php(746): MediaWiki->performRequest()
#10 /vagrant/mediawiki/includes/MediaWiki.php(520): MediaWiki->main()
#11 /vagrant/mediawiki/index.php(43): MediaWiki->run()
#12 /var/www/w/index.php(5): require(string)
#13 {main}
```
See also {T110696}.