When an edit is reverted, and the previous edit is rev-deled, the username for the edit summary is "null." Instead, it should just be "Reverted 2 edits" or something to that effect.
For example: https://en.wikipedia.org/w/index.php?title=User_talk:Reaper_Eternal&curid=24473975&diff=546170158&oldid=546169945 (User:Null did not make the edit, I confirmed with two en.wp admins)
Version: 1.21.x
Severity: minor
Well, "null" is better than exposing the hidden username, but I agree that links to [[Special:Contributions/Null]] and [[User talk:Null]] there are a bit bizarre and wrong. :-)
Are we sure this isn't a local customized message? Is this possible to reproduce on test.wikipedia.org or some other site with fewer customized messages?
There is message revertpage and revertpage-nouser, where the later is for the RevDel case. But the summary of your link contains a link to Twinkle, so looks like that tool is making damage here.
INVALID, because the bug is in Twinkle.
MediaWiki has a seperate message for rollback and with bug 48571 now for undo.