关于 修改UITextview内容遇到问题

从API中获取数据，然后将数据写入UITextview。但是系统报出错误。

下面是读取写入数据的代码：

NSOperationQueue *apiCallsQueue = [[NSOperationQueue alloc] init];
NSString *urlstring=@"x.com/api.php?id=";
urlstring =[urlstring stringByAppendingString:self.newsId];
NSURL *newsdetail = [NSURL URLWithString:urlstring];
newsdetailrequest = [NSURLRequest requestWithURL:newsdetail];

@try
{
    [NSURLConnection sendAsynchronousRequest:newsdetailrequest queue:apiCallsQueue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
    {
        NSDictionary *dictionary =  [NSJSONSerialization JSONObjectWithData:data options:nil error:nil];
        [textview setText:[dictionary objectForKey:@"title"]];
    }];
}
@catch (NSException *exception)
{
    UIAlertView *errorView = [[UIAlertView alloc] initWithTitle:@"Error"
                                                        message:@"Error"
                                                       delegate:nil
                                              cancelButtonTitle:@"Tamam"
                                              otherButtonTitles:nil];
    [errorView show];
}

错误输出：

2013-06-18 16:37:16.214 xpro[6816:5103] bool _WebTryThreadLock(bool), 0x7142ee0: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...
1   0x3571fe9 WebThreadLock
2   0x15846e -[UITextView setText:]
3   0x5f00 __39-[NewsDetailViewController viewDidLoad]_block_invoke
4   0xbdbcf8 __block_global_0
5   0xb4375a -[NSBlockOperation main]
6   0xb11453 -[__NSOperationInternal start]
7   0xb11164 -[NSOperation start]
8   0xb9da31 __block_global_6
9   0x49ff53f _dispatch_call_block_and_release
10  0x4a11014 _dispatch_client_callout
11  0x4a022e8 _dispatch_root_queue_drain
12  0x4a02450 _dispatch_worker_thread2
13  0x96c53e72 _pthread_wqthread
14  0x96c3bd2a start_wqthread
(lldb) 

controller.h文件：

@interface NewsDetailViewController : UIViewController
{
NSURLRequest *newsdetailrequest;
NSMutableData *newsdetailData;
}
@property (weak, nonatomic) IBOutlet UINavigationBar *newsdetailTitle;
@property (weak, nonatomic) IBOutlet NSString *navBarTitle;
@property (weak, nonatomic) IBOutlet NSString *newsId;
@property (weak, nonatomic) IBOutlet UITextView *htmlDetail;