Fail early if we're passed a null chain callback.
Fail early if we're passed a null chain callback.

rachelharvey/playn,longtaoge/playn,rachelharvey/playn,tinkerstudent/playn,ruslansennov/playn2,playn/playn,playn/playn,ruslansennov/playn2,playn/playn,longtaoge/playn,thecocce/playn-1,tinkerstudent/playn,thecocce/playn-1