故事板视图不会出现

Question

我正试图从故事板中呈现一个视图，这是我viewDidLoad的样子：

- (void)viewDidLoad
{
    [super viewDidLoad];

    // I'm doing some stuff here...

    UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard" 
        bundle:nil];
    UIViewController *rvc = [storyboard 
        instantiateViewControllerWithIdentifier:@"RVC"];
    [rvc setModalPresentationStyle:UIModalPresentationFullScreen];
    [self presentModalViewController:rvc animated:YES];

    //[self performSegueWithIdentifier:@"pushRVC" sender:self];
}

代码似乎运行良好，因为没有崩溃，但视图不会出现。它也不适用于performSegueWithIdentifier。

奇怪的是，在我使用NSURLConnection连接到PHP页面之前，它工作正常。

以下代码没问题，我没有遇到任何问题：

- (void)viewDidLoad
{
    [super viewDidLoad];

    // I'm doing the same stuff here...

    NSMutableData *responseData = [[NSMutableData alloc] init];                
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL 
        URLWithString:@"myURL"]]; 
    NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:request 
        delegate:self];
    [connection start];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    NSString *responseString = [[NSString alloc] initWithData:responseData
        encoding:NSUTF8StringEncoding];

    NSError *jsonError;
    SBJsonParser *jsonParser = [[SBJsonParser alloc] init];
    NSDictionary *jsonResults = [jsonParser objectWithString:responseString 
        error:&jsonError];

    if (jsonResults == nil) {
        NSLog(@"%@", [jsonError localizedDescription]);
    }
    else {
        // Messing around with the results...
    }        

    UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard" 
        bundle:nil];
    UIViewController *rvc = [storyboard instantiateViewControllerWithIdentifier:@"RVC"];
    [rvc setModalPresentationStyle:UIModalPresentationFullScreen];
    [self presentModalViewController:rvc animated:YES];

    //[self performSegueWithIdentifier:@"pushRVC" sender:self];
}

这对我来说很奇怪，不知道它可能是什么？

提前致谢。