Problem with an Open Source mod that I want to fix ...

[OrgeAlexj06]

Hello,
I recently downloaded a mod from Jobs in 1.12.2 but I am experiencing a bug. When I put it in the mods folder and start my server everything works fine, but when I restart the server, when I try to open the jobs gui, I kick the server with this error in the logs : https://pastebin.com/SHL4sXJn
Here are the classes affected by the error
GainXPUtil: https://pastebin.com/yVBrKNkd
CommonEvents: https://pastebin.com/2USQPnaf
This mod is open source on GitHub, which is why I have access to the code and permission to modify it.
If anyone could help me solve this problem that would be great.
If you misunderstood the source of my problem, you can put your Discord nickname in and I'll add you to explain more in depth.
Thank you