今天把Xcode升级了，模拟器 用的12.1的系统，运行时发现项目总是崩溃，采用9.3系统的测试机发现错误日志如下：
Application windows are expected to have a root view controller at the end of application launch
解决办法如下：
self.window = [[UIWindow alloc]initWithFrame:SCREEN_RECT]; self.window.backgroundColor = [UIColor whiteColor]; self.window.rootViewController = [[UIViewController alloc]init]; [self.window makeKeyAndVisible];
Application windows are expected to have a root view controller at the end of application launch