Changing from peaceful to easy, program crashed

this problem happened using the FTB Mod
In options during the game I was changing from 'peaceful' to 'easy' to spawn mobs when it crashed

Steps to Reproduce:
Can't get back into the game because now on opening the world, it crashes!
Sending 2 crash reports
1st crash was the initial one. 2nd crash is my attempt to get back into the game.