If you try to lock a topic (e.g. in beta labs http://en.wikipedia.beta.wmflabs.org/wiki/Talk:Flow or mw.org https://www.mediawiki.org/wiki/Talk:Sandbox) and click "Preview" after insert a test reason, you will get a red warning "The content parameter must be set" and don't see the reason preview. Same for unlock topic in Firefox and Chrome.
Version: master
Severity: normal