It's a common issue with the RPG Maker itself, not specifically to my plugin. It happens since MV. When you install a new plugin, that stores changes on the save file, you need to start a new game, otherwise you get a lot of errors. Some example of other people talking about it:
- Dealing with Conflicts (can't read x of undefined) | RPG Maker Forums
- Question about RPG MV Save files and new plugins affecting them. : r/RPGMaker
- Summon Actor | Page 2 | RPG Maker Forums
- MV - TypeError Cannot set property '0' of undefined | RPG Maker Forums
- MV and Save Files | RPG Maker Forums
The save file does not understand when you do that, when you load it with a new plugin(that creates some data to be saved) that previous it did not have. Unfortunatelly you will need to start a new game to make it work.
BUT, I will see if somehow nowdays I can try to make a workaround for that. This is a common issues for years, since MV. Maybe there is a way to solve that... But I can promise you when I will do that.