Fix RCTEventDispatcher Crash

Summary:
This crash was introduced in D22962320 (ffdfbbec08), which landed 8/17 and went into 8/23 cut. I plan to pick this diff into that release.

I can't repro this, but there must be a scenario (in bridged mode), where we try to flush event queue while bridge is tearing down. I plan to consider this scenario going forward.

Changelog: [Internal]

Reviewed By: JoshuaGross

Differential Revision: D23306445

fbshipit-source-id: 285d4b94a17423c3b08d83e7041c4ee04b7e6d0c

React/Base/RCTEventDispatcher.m

@@ -179,7 +179,7 @@ RCT_EXPORT_MODULE()
             [self flushEventsQueue];
           }
                   queue:RCTJSThread];
-    } else {
+    } else if (_dispatchToJSThread) {
       _dispatchToJSThread(^{
         [self flushEventsQueue];
       });