iphone program received signal:"SIGABRT"


UITabBarController *tabBarController =  
    (UITabBarController *)self.window.rootViewController;  
    UINavigationController *navigationController =  
    [[tabBarController viewControllers] objectAtIndex:0];  
    PlayerViewController *playerViewController =  
    [[navigationController viewControllers] objectAtIndex:0];
    playerViewController.players = players;  
    return YES;用的Stroryboard
TabBar->Nav(包含TableView)照着示例写的，可是运行报错。
报错：iphone program received signal:"SIGABRT"
位置：
PlayerViewController *playerViewController =  
    [[navigationController viewControllers] objectAtIndex:0];