Thanks for the report! The crash occurred because the dummy character has been destroyed. Why? Apparently, "Wave Defense" deletes all surfaces except "nauvis" on several occasions (e.g. if a player joined the game, difficulty is changed, or a new round is started), and everything on them. While I already attempt to restore missing preview and dummy characters, this fails if I try to recreate the characters on a surface that doesn't exist. I simply didn't reckon with somebody removing everything.
I'll put in some safeguards, to immediately restore preview/dummy surfaces when they are destroyed by another mod. However, it would be better by far if I could get the devs to modify the function deleting the surfaces, so that it ignores mine.
