There are two fighter lizards in a cave. When I click on end fight, I get the error below.
End fight as in end turn?
Still trying to recreate your issue but with little luck so far. The only way I can see this happening is if you had 0 alive players yet were still playing for whatever reason. Or maybe your last player dies shortly before the lizardmen attack?
Have you tried reloading a save before the battle takes place?