When trying to access Special:Preferences after updating to the release candidate of 1.30, I am getting a MW exception. Tagging as high priority as it seems important.
Version: 1.30 RC
PHP: 5.6
```
2017-12-02 19:03:45 test1 test1wiki: [e74b9347da1dfd50d354570f] /wiki/Special:Preferences MWException from line 148 of /srv/mediawiki/w/includes/Preferences.php: Global default '' is invalid for field theme
#0 /srv/mediawiki/w/includes/Preferences.php(100): Preferences::loadPreferenceValues(User, RequestContext, array)
#1 /srv/mediawiki/w/includes/Preferences.php(1356): Preferences::getPreferences(User, RequestContext)
#2 /srv/mediawiki/w/includes/specials/SpecialPreferences.php(127): Preferences::getFormObject(User, RequestContext)
#3 /srv/mediawiki/w/includes/specials/SpecialPreferences.php(84): SpecialPreferences->getFormObject(User, RequestContext)
#4 /srv/mediawiki/w/includes/specialpage/SpecialPage.php(522): SpecialPreferences->execute(NULL)
#5 /srv/mediawiki/w/includes/specialpage/SpecialPageFactory.php(578): SpecialPage->run(NULL)
#6 /srv/mediawiki/w/includes/MediaWiki.php(287): SpecialPageFactory::executePath(Title, RequestContext)
#7 /srv/mediawiki/w/includes/MediaWiki.php(851): MediaWiki->performRequest()
#8 /srv/mediawiki/w/includes/MediaWiki.php(523): MediaWiki->main()
#9 /srv/mediawiki/w/index.php(43): MediaWiki->run()
#10 {main}
```