The only way that NPE is possible is if the game’s entire combat engine is null, which I didn’t think would be possible expect for the very first frame. Can you give me more info about the crash?
Is it happening as soon as you load in, or before the save can finish loading?
Are you opening the refit screen manually or no?
Did you press a button right before the crash happened, and if so, which button?
Edit:
I think I've been able to reproduce your bug. To fix, instead of going directly to refit screen from market ('R'), just enter the fleet screen first ('F') and then go to the refit screen. I'll push a hotfix as well.