Application windows are expected to have a root view controller at the end of application launch

今天用XCode建立Empty Application时遇到了这样一个错误：

Application windows are expected to have a root view controller at the end of application launch

原因是XCode 5以前有一个根视图的概念，没有找到会导致错误，所以只需要为该窗体创建一个根式图即可：

self.window.rootViewController = [[UIViewController alloc] init];