Xcode11.1新生成项目黑屏问题：[Application] The app delegate must implement the window property if it wants ...

解决方案：
在AppDelegate.m中添加代码，如下：

@implementation AppDelegate
// 添加这行代码
@synthesize window = _window;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    return YES;
}