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

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];