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

今天把Xcode升級了，模擬器 用的12.1的系統，運行時發現項目總是崩潰，采用9.3系統的測試機發現錯誤日志如下：

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

解決辦法如下：

self.window = [[UIWindow alloc]initWithFrame:SCREEN_RECT];
self.window.backgroundColor = [UIColor whiteColor];
self.window.rootViewController = [[UIViewController alloc]init];
[self.window makeKeyAndVisible];