I am aware this may be a duplicate, but I think the problem is having a custom villager with an 'Unusable Trade'.

It appears that custom villager trading can work without the unusable trade, but I'm not 100% positive.

Anyway, create a custom villager using MC Edit. Except tick the box that says 'Unusable trade'. Then open minecraft and trade with that villager, it crashes the game.

I'm worried that I can't play Uncharted Territory 3 by AMLUP because of this, I hope this can be fixed in 1.7.3