This happens on some old changesets where there wouldn't be any relevant results anyway, but the error is currently shown not just in the console, it is shown on the page as well. I'm reporting that it is shown to users, not that it has no results (that seems fine).
https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Kartographer/+/304151
[wm-checks-api] error TypeError: changeMessage.real_author is undefined @ accept() @ filterMessages()