For any bug reports please provide the following details as well,

• MC Version: 1.12.2
• C&B Version: 14.18
• Do You have Optifine: No

Crash Log

Loaded a world and tried to open my inventory.

I don't think that this is caused by C&B, something is messing with the creative tab list in multiple threads, it just so happened that C&B was in the main thread and was busy updating the list when after something else did something.

That doesn't really help track down the cause, but since C&B isn't using threads here its really the only explanation I have.

It didn't repeat when I reloaded the world, so that's good. I'll see what I can figure out (if anything).