uialertaction 中 AVQuery 不执行

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NSString *userID = @"userid123";
UIAlertController *alertController = [UIAlertController alertControllerWithTitle:nil message:@"确定删除？" preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *cancelAction = [UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:nil];
UIAlertAction *giveUpAction = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDestructive handler:^(UIAlertAction *action) {
    AVQuery *query = [[AVQuery alloc] initWithClassName:@"ToDo"];
    [query whereKey:@"user_id" equalTo:userID];
    NSError *err;
    NSArray *objects = [query findObjects:&err];
    if (!err) {
        [AVObject deleteAllInBackground:objects block:^(BOOL succeeded, NSError *error) {
            if (!error) {
                NSLog(@"delete done.");
            }
            else {
                NSLog(@"error info: %@", error);
            }
        }];
    }
    else {
        NSLog(@"error info: %@", err);
    }
}];
[alertController addAction:cancelAction];
[alertController addAction:giveUpAction];
[self presentViewController:alertController animated:YES completion:nil];

上面的代码执行到NSArray *objects = [query findObjects:&error];后就结束了，没有任何输出；
断点设置在if (!err) {处，但程序不会执行到此处。

请问这是什么情况呢？是因为线程问题吗？