Fix NPE on children expansion
Fix NPE on children expansion

Summary: See task for stack trace.

Reviewed By: timur-valiev

Differential Revision: D30393458

fbshipit-source-id: 71589b42fd579db95dfb04a96a00966f8bc65a2a

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper