Ensure worker factory is not null in forEachWorker
Ensure worker factory is not null in forEachWorker

Summary: Null check workerFactory_ in forEachWorker to ensure it has been initialized.

Reviewed By: shamdor-fb

Differential Revision: D3547878

fbshipit-source-id: 559754c15e27be52ba2852c2bd215bdcfa0a3ff2

facebook/wangle,facebook/wangle,facebook/wangle