Client keep crashing quite often, last time was when i q-ed some bows from a chest,before that,it crashed when i cleared the NEI search bar with right click.

Modpack version : 1.2a

Crash Report

https://pastebin.com/mH60WC6e
Edit:It looks it keeps crashing every few minutes randomly.

It appears to be due to a Pech spawning, so your mobfarm is likely in a Magical Forest biome.
Best you can do right now, is to disable Pechs in the Thaumcraft config