Hi there!
thanks for the detailed explanation!
I believe the problem is the same with or without visustella: After you insert my plugin into your project, you need to start a new game. If you try to continue a previous game, where this plugin did not exist, it will throw an error. Since my plugin stores information on the save file, if you choose to continue a previous save file where it did not exist, when you load that save file it will try to look at the information that would be stored, and it will not find it.
Try starting a new game after you put that plugin and let me know if any other error will arise.