Internal Exception: java.lang.nullpointerexception

So I setup a RLCraft Modpack java server, no issues through the entirety of the making of it until I attempted to join the server that I ported through himachi and then I got this error: Internal Exception: java.lang.nullpointerexception. My client for Minecraft is Forge 1.12.2 14.23.5.2803, the same version that the server runs on.

I've tried what seems nearly a hundred different tutorials to fix the error and come to no success every time, I would greatly appreciate someone tell me what to do to fix this issue. I'm really new to this site so I have no clue what files I can upload to give info on whats causing it.