RCTBridge required dispatch_sync to load RCTDevLoadingView. This may lead to deadlocks

I fixed it with updating AppDelegate.m as below:

   #if RCT_DEV
   #import <React/RCTDevLoadingView.h>
   #endif
   //...

   - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
   {
   //...
   RCTBridge *bridge = [[RCTBridge alloc] initWithBundleURL:jsCodeLocation
                 moduleProvider:nil
                  launchOptions:launchOptions];
   #if RCT_DEV
   [bridge moduleForClass:[RCTDevLoadingView class]];
   #endif

   RCTRootView *rootView = [[RCTRootView alloc] initWithBridge:bridge
                    moduleName:@"<AddYourAppNameHere>"
                 initialProperties:nil];
   //...
   }

Source:
grossingdev’s answer Dec 10, 2017 from: https://github.com/facebook/react-native/issues/16376