Crash trading with a custom villager: java.lang.NullPointerException: Cannot invoke "cdp.S_()" because the return value of "cdt.c()" is null

Not sure what went wrong but it crashed when I was trading with a custom Villager and move the mouse to one trade element before the Villager refreshed his trade.

If you can find out what went wrong and that was indeed a bug, pls help me retitle this issue.