Fixes the fix of wrong completionHandler call

Unfortunately the last fix introduces a new bug! The call for `[[UIApplication sharedApplication] registerForRemoteNotifications]` *must* be handled on main thread! This is fixed now.

permission_handler/ios/Classes/strategies/NotificationPermissionStrategy.m

@@ -38,8 +38,10 @@
           return;
         }
 
+        dispatch_async(dispatch_get_main_queue(), ^{
             [[UIApplication sharedApplication] registerForRemoteNotifications];
             completionHandler(PermissionStatusGranted);
+        });
       }];
 
     } else {