Forge version: Doesn't matter (i think)
Twilight Forest version: 13.20.1.2429
Link to crash log:
crash-2017-08-06_14.29.37-client.txt (sorry u might not want this but idk how to make things a link. let me know if i did a good)
This was also by ZombieEnderman5, not me. he is a kewl person on discord.

Tried to open my inventory with a Maze Map.

They also said these wonderful things =)

Something to do with ItemTFMagicMap and ItemMap and GuiScreen
It is the same version. Same world in fact.
No idea if it helps but it crashed right after I collected a piece of Raw Meef
Huh... like I thought, the unknown map bug only occurs in chests @hammer Bro (Me in nickname form)

Happens with Vanilla maps as well, I can't exactly reproduce it consistently. Seems kinda random.

It's because we're doing a dirty hack piggybacking off of vanilla's mapdata ID
Needs this to be fixed MinecraftForge/MinecraftForge#4068

rolling into #29