移动推送 iOS SDK: Main Thread Checker: UI API called on a background thread

Xcode 升级到 Xcode 9后，集成时若提示下述错误：

1. [backcolor=transparent]Main[backcolor=transparent] [backcolor=transparent]Thread[backcolor=transparent] [backcolor=transparent]Checker[backcolor=transparent]:[backcolor=transparent] UI API called on a background thread

1. [backcolor=transparent][[backcolor=transparent]_notificationCenter requestAuthorizationWithOptions[backcolor=transparent]:[backcolor=transparent]UNAuthorizationOptionAlert[backcolor=transparent] [backcolor=transparent]|[backcolor=transparent] [backcolor=transparent]UNAuthorizationOptionBadge[backcolor=transparent] [backcolor=transparent]|[backcolor=transparent] [backcolor=transparent]UNAuthorizationOptionSound[backcolor=transparent] completionHandler[backcolor=transparent]:^([backcolor=transparent]BOOL granted[backcolor=transparent],[backcolor=transparent] [backcolor=transparent]NSError[backcolor=transparent] [backcolor=transparent]*[backcolor=transparent] [backcolor=transparent]_Nullable[backcolor=transparent] error[backcolor=transparent])[backcolor=transparent] [backcolor=transparent]{
2. [backcolor=transparent]    [backcolor=transparent]if[backcolor=transparent] [backcolor=transparent]([backcolor=transparent]granted[backcolor=transparent])[backcolor=transparent] [backcolor=transparent]{
3. [backcolor=transparent]        [backcolor=transparent]// granted
4. [backcolor=transparent]        [backcolor=transparent]NSLog[backcolor=transparent](@[backcolor=transparent]"User authored notification."[backcolor=transparent]);
5. [backcolor=transparent]        [backcolor=transparent]// 向APNs注册，获取deviceToken
6. [backcolor=transparent]        [backcolor=transparent][[backcolor=transparent]application registerForRemoteNotifications[backcolor=transparent]];
7. [backcolor=transparent]    [backcolor=transparent]}[backcolor=transparent] [backcolor=transparent]else[backcolor=transparent] [backcolor=transparent]{
8. [backcolor=transparent]        [backcolor=transparent]// not granted
9. [backcolor=transparent]        [backcolor=transparent]NSLog[backcolor=transparent](@[backcolor=transparent]"User denied notification."[backcolor=transparent]);
10. [backcolor=transparent]    [backcolor=transparent]}
11. [backcolor=transparent]}];

1. [backcolor=transparent][[backcolor=transparent]_notificationCenter requestAuthorizationWithOptions[backcolor=transparent]:[backcolor=transparent]UNAuthorizationOptionAlert[backcolor=transparent] [backcolor=transparent]|[backcolor=transparent] [backcolor=transparent]UNAuthorizationOptionBadge[backcolor=transparent] [backcolor=transparent]|[backcolor=transparent] [backcolor=transparent]UNAuthorizationOptionSound[backcolor=transparent] completionHandler[backcolor=transparent]:^([backcolor=transparent]BOOL granted[backcolor=transparent],[backcolor=transparent] [backcolor=transparent]NSError[backcolor=transparent] [backcolor=transparent]*[backcolor=transparent] [backcolor=transparent]_Nullable[backcolor=transparent] error[backcolor=transparent])[backcolor=transparent] [backcolor=transparent]{
2. [backcolor=transparent]    [backcolor=transparent]if[backcolor=transparent] [backcolor=transparent]([backcolor=transparent]granted[backcolor=transparent])[backcolor=transparent] [backcolor=transparent]{
3. [backcolor=transparent]        [backcolor=transparent]// granted
4. [backcolor=transparent]        [backcolor=transparent]NSLog[backcolor=transparent](@[backcolor=transparent]"User authored notification."[backcolor=transparent]);
5. [backcolor=transparent]        [backcolor=transparent]// 向APNs注册，获取deviceToken
6. [backcolor=transparent]        dispatch_async[backcolor=transparent]([backcolor=transparent]dispatch_get_main_queue[backcolor=transparent](),[backcolor=transparent] [backcolor=transparent]^{
7. [backcolor=transparent]            [backcolor=transparent][[backcolor=transparent]application registerForRemoteNotifications[backcolor=transparent]];
8. [backcolor=transparent]        [backcolor=transparent]};
9. [backcolor=transparent]    [backcolor=transparent]}[backcolor=transparent] [backcolor=transparent]else[backcolor=transparent] [backcolor=transparent]{
10. [backcolor=transparent]        [backcolor=transparent]// not granted
11. [backcolor=transparent]        [backcolor=transparent]NSLog[backcolor=transparent](@[backcolor=transparent]"User denied notification."[backcolor=transparent]);
12. [backcolor=transparent]    [backcolor=transparent]}
13. [backcolor=transparent]}];