Summary:Unmounting the RefreshControl during onRefresh callback causes the native ref to become null, this simply adds a null check to prevent the crash.
Closes https://github.com/facebook/react-native/pull/6931
Differential Revision: D3167637
fb-gh-sync-id: c2420b7a3b672d62dd349a6d35bb05399a00620c
fbshipit-source-id: c2420b7a3b672d62dd349a6d35bb05399a00620c