To Reproduce
Steps to reproduce the crash:

1. Place WW2 Armor Box
2. Enter GUI
3. Click Right arrow in GUI twice

Versions (please complete the following information)

• Forge: 14.23.5.2772
• Flan's Mod: 5.5.1&5.5.2 (Tried both versions)

Crash Log
The game crashed whilst rendering screen
Error: java.lang.NullPointerException: Rendering screen

It seems you are using an outdated version. Please follow the instructions at https://github.com/FlansMods/FlansMod/wiki/Installing-FlansMod to install the latest version