No it's because it tries to load "$game", which does not exist with the default engine. What this object contains is all the default classes into one object (eg. $game.party, $game.map instead of $game_party and $game_map as separate objects). That is why this issue occurs, but I don't see where these things even occur, I fixed all the parts to use "$game_" instead of "$game.", so it doesn't even show up. I mean, "$game.party" isn't in the script at all, so I don't get why it tries to access this unavailable object for no apparent reason. I'm too tired to look into it. I don't have the original, only the modification to work with the Game:Class, which you don't use and would require a complete rewrite of all your scripts.
EDIT: Problem found. It lies in the "refresh" method where you find "game.map" and "game.party". This is the incorrect data structure. I'm too tired to solve it for you, but basically you need to get the original loading structures and make sure you load those two objects from the save file selected and obtain them under the same variables. In addition, as said, the loading and saving methods as mentioned before needs to match the saving and loading structure of the default. You find these by doing Ctrl+Shift+F and searching for "write_save_data" and "read_save_data" and checking the default scripts. Just follow those leads.