I could reproduce twice, so I am 99% sure that the title is correct.
Quark crashes when in creative inventory and an item is hovered over a shulker box. Not reproducible in 1.15, so issue is in the ported version.

Expected:

Game screen right before crash (Not Responding):

Crash Report here.

Another link: https://gist.github.com/SwanX1/8bcb60c01d5c47d4defad82b1541cd17#file-crash-2020-07-21_17-18-53-client-txt

Forge Version: 32.0.71
AutoRegLib Version: 1.6-41
Quark Version: 2.2-252
All versions are the latest ones as of writing.