IGNORE ALL THIS, READ OTHER POST HOW TO ACCURATELY REPRODUCE

I have no idea what exactly triggers it (I know that you need, at least, that tab open), just have a video here until it bugs:

To Reproduce
Steps to reproduce the behavior:
ATM10 2.8 Modpack: https://www.curseforge.com/minecraft/modpacks/all-the-mods-10/files/6009022
World with bugged backpack: Soph Backpacks.zip

1. Open backpack with right click
2. Crash

If you want to workaround this crash, before opening the bugged backpack, grab any empty backpack, open, now you can open the other.

Versions
Backpack: 3.20.27.1154
Core: 1.0.2.810

Updating, just managed to reproduce with only backpack mods and crafting tweaks, just do this:

grab a backpack
grab a crafting upgrade
add the upgrade to backpack
open the right side tab, left it open
leave world and:
CLOSE MINECRAFT, YOU NEED TO CLOSE THE GAME

Now join again and do the following steps:

Open Inv
Shift to preview
Click to open
It will crash

Thanks for the detailed report. I have just released a new version of core that has the fix for this.