Describe the bug
Unfortunately, I was not paying attention to my screen when the crash occurred, so I'm not positive what caused it, but I might have accidentally pressed B to open the backpack while my inventory was already open.
Crash Report

To Reproduce
Steps to reproduce the behavior:

1. Open Inventory
2. Press B to open backpack
3. Crash?

Expected behavior

• No Crashes

Versions
Minecraft v1.16.5
Forge v36.1.32
Sophisticated Backpacks v2.5.0.271

So this is not possible to recreate with just SBP nor in pack that I play (Seaopolis). You have a mod in there that changes MC logic of checking whether keybind can be pressed in the given gui (KeyConflictContext). If you can figure out which mod it is I can take a look at what they are doing just to double check there really isn't an issue in sbp, but very likely you will then need to report this issue to that mod as something making MC ignore key conflict context would break many mods and may even break some vanilla keybind resolution especially if you use the same key for two different actions that can only every trigger in different contexts.

just looking at the mods list you may want to start with JEK as that seems to mess with keybinds and possibly has an issue there

It may have been JEK, I had removed that mod recently and did not get a crash when pressing B while in my inventory. Thanks for looking into it.

alright, thanks for confirming closing this