message: waterfall fix element unfind
diff: @@ -109,7 +109,7 @@ export default function(type) {
  unbind(el) {
  const context = el[CONTEXT];
  - context.scrollEventTarget.removeEventListener('scroll', context.scrollEventListener);
  + context.scrollEventTarget && context.scrollEventTarget.removeEventListener('scroll', context.scrollEventListener);
  }
  };
  };
  