Invoke registerForRemoteNotifications on main UI thread to avoid error messages in Xcode

Summary:
Invoke registerForRemoteNotifications on main UI thread to avoid error messages in Xcode.

Changelog:
[iOS][Fixed] - Invoke registerForRemoteNotifications on main UI thread.

Reviewed By: philIip

Differential Revision: D33780141

fbshipit-source-id: 64b825dfc15e7ac262e210b90bb334a7e415e402

Author: Moses DeJong

Libraries/PushNotificationIOS/RCTPushNotificationManager.mm

@@ -313,10 +313,12 @@ - (void)handleRemoteNotificationRegistrationError:(NSNotification *)notification
     if (error != NULL) {
       reject(@"-1", @"Error - Push authorization request failed.", error);
     } else {
-      [RCTSharedApplication() registerForRemoteNotifications];
-      [UNUserNotificationCenter.currentNotificationCenter getNotificationSettingsWithCompletionHandler:^(UNNotificationSettings * _Nonnull settings) {
-         resolve(RCTPromiseResolveValueForUNNotificationSettings(settings));
-      }];
+      dispatch_async(dispatch_get_main_queue(), ^{
+        [RCTSharedApplication() registerForRemoteNotifications];
+        [UNUserNotificationCenter.currentNotificationCenter getNotificationSettingsWithCompletionHandler:^(UNNotificationSettings * _Nonnull settings) {
+           resolve(RCTPromiseResolveValueForUNNotificationSettings(settings));
+        }];
+      });
     }
   }];
 }