Minecraft 1.12.2
Forge 1.12.2-14.23.5.2854

I got this strange crash on my client (it wont straight closed the game due to VanillaFix) when other player are playing with me, in both LAN or Dedicated server worlds.

Crash report # 1 (without VanillaFix it would crash the game): https://hastebin.com/akelelafuc.sql
Crash report # 2 (since VanillaFix doesnt allow the game to close, it keeps running and this other crash happens): https://hastebin.com/oxebetiraf.rb

Regardless, in server-side (when playing on a dedicated server) no crash happens, its just in clients.

Although VanillaFix can have some play on this, i suspect Midnight is the main source of the issue: it happens when its portals appears near the players.