A similar error to this case has been fixed:
In the text for reaction on comment received, in the Last activity page, "your comment" was not actually a link. So this was fixed too.Use case:
Forum allows guest / unregistered posts, however to appear they require moderator approval.
When I try to move a post which has comments by guests to a new thread I get an error "Attempted to convert NULL to string/binary [username] ".