Hm... That's strange, I've tested it a lot the last patch to prevent this kind of crash errors :/
Usually, this message:
Unable to find instance for object index -4 at gml_Script_scr_player_turn
means that the game tried to localize an idol in the field to realize an action during your turn, like moving or attacking, but the game itself lost the target of said idol within the process. This used to happen before the last patches when you realize quick actions and change the selected idol, like attacking an enemy and instantly changing the selected idol with the a hotkey, or moving to a tile and then passing the turn with Space Bar before it finishes the movement.
Perhaps this was the case? Also, were you using the free mode camera, or was it locked?
I'll try to figure it out to patch it in the future. I hope this isn't affecting your experience overall.
Again, thank you very much for your support! I really hope this become not only a good Hololive fangame, but a good strategy game, so it makes me really glad to hear that! ^^