Forge version: 36.1.2
Twilight Forest version: 4.0.398
Link to crash log: https://pastebin.com/1QSwxCb2

Steps to reproduce:

1. Start Minecraft
2. Generate a world

What I expected to happen:
Go to the world

What happened instead:
After loading and seeing 1 chunk it says Saving game and Minecraft crashes

Yeah, my bad. Working on a fix right now. Sorry!

Actually, looking at the crashlog, you seem to have optifine installed. Please get rid of optifine, or refer to #1017 to fix it.