Seen this one before but I'm struggling to recreate it. I'd need the traceback save or exact steps what lead you to the error.
By traceback save I mean the save file that is created when you get the error. Either that or information on how I can recreate it - what spells you used, who's in your party, who are you fighting and so on. Anything you were doing that could lead to the error.
Sadly, the error code isn't that helpful on its own. It's caused by one of your characters not being in the display list of player characters, but why isn't the character there is the real issue.