I get this error when trying to load a save from my project.

Everything was working before. Game is able to start a new game and save, but trying to continue crashes the project. Any solutions?
YEP_CoreEngine.js:1098 TypeError: $gameSystem.versionId is not a function
at Scene_Load.reloadMapIfUpdated (rpg_scenes.js:1784)
at Scene_Load.onLoadSuccess (rpg_scenes.js:1773)
at Scene_Load.onSavefileOk (rpg_scenes.js:1764)
at Window_SavefileList.Window_Selectable.callHandler (rpg_windows.js:902)
at Window_SavefileList.Window_Selectable.callOkHandler (rpg_windows.js:1171)
at Window_SavefileList.Window_Selectable.processOk (rpg_windows.js:1156)
at Window_SavefileList.Window_Selectable.processHandling (rpg_windows.js:1027)
at Window_SavefileList.Window_Selectable.update (rpg_windows.js:984)
at rpg_core.js:7035
at Array.forEach (<anonymous>)