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

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

解决方法：在ios5以上版本，应用程序加载时，需要一个 root view controller ，所以需要编写代码

_rootViewController = [[RootViewController alloc] init];

self.window.rootViewController = _rootViewController;