iOS: guard against bad RCTModuleData instance

Summary: In some rare race condition (usually involving network request handling vs bridge shutting down), there may be bad access to an RCTModuleData that may have been de-allocated. To prevent crashes, let's guard the access and return nil appropriately.

Reviewed By: yungsters

Differential Revision: D13548755

fbshipit-source-id: b97326524cd9ca70a13d15098a1eaadfc7f1a6a8

React/CxxBridge/RCTCxxBridge.mm

@@ -458,6 +458,12 @@ struct RCTInstanceCallback : public InstanceCallback {
 
   RCTModuleData *moduleData = _moduleDataByName[moduleName];
   if (moduleData) {
+    if (![moduleData isKindOfClass:[RCTModuleData class]]) {
+      // There is rare race condition where the data stored in the dictionary
+      // may have been deallocated, which means the module instance is no longer
+      // usable.
+      return nil;
+    }
     return moduleData.instance;
   }