Here is an issue submitted from a user that I was able to replicate the same way.

When attempting to apply an enchantment from an Enchanted Book to some Wither Leggings, the game crashes.

Expected Behavior
The Wither Leggings should be able to have the enchantment applied to them and then removed from the anvil.

Current Behavior
Placing the Wither Leggings in an Anvil doesn't crash the game, but adding an Enchanted Book will, thus preventing you from applying the enchantment and crashing the game.

Steps to Reproduce
Procure a pair of Wither Leggings from the Tiny Progressions mod and an Enchanted Book (I was using a Protection IV book).
Place the Wither Leggings in an Anvil, and then place the Enchanted Book in.
Log
https://gist.github.com/TheWooper/4a939a213f4c0940d19adf64390cc026

AllTheMods/ATM-5#175

I will look into the issue.

There's an error in this line. Function getRepairMaterial tries to get uninitialized variable, so NullPointerException is thrown.

Thanks, sorry long work hours.. and this last week has been long.. I see about fixing it maybe have a fix this weekend.

Sorry the wait on this.. and update is going out today for version 4.0.6 that has the fix for this.