To Reproduce
Steps to reproduce the crash:

1. Place a Armour/weapon crafting box
2. open it and press the arrow twice
3. then crashes

Screenshots
(https://user-images.githubusercontent.com/55502292/65189318-95f1f380-da3e-11e9-92f4-bf2c6c2c56f3.png)
(https://user-images.githubusercontent.com/55502292/65189406-d3568100-da3e-11e9-8e43-da5b7af1da55.png)

Versions (please complete the following information)

• Forge: forge-1.12-14.21.1.2443
• Flan's Mod: 5.5.1 for MC 1.12.2

Crash Log
(https://github.com/FlansMods/FlansMod/files/3628521/crash-2019-09-18_18.05.27-client.txt)

Additional context
probably crashes in creative but i put what i was doing in the steps

I was using vanilla fix so it didn't really crash but it still says its a crash
and to report this

Version 5.5.1 is out of date and no longer supported. Please try using the latest FlansMod version, available here: https://github.com/FlansMods/FlansMod/releases

Duplicate of #854.
The issue was already fixed in d02694a, to get the fix you need to do what @Liruxo said.