XCode11中The app delegate must implement the window property if it wants to use 如何解决

用XCode11新建一个工程支持的最小iOS版本小于iOS13的话，XCode控制台会爆出[Application] The app delegate must implement the window property if it wants to use a main storyboard file.的提示。只需要两步就可以解决问题，

1.添加window属性

@property (nonatomic,strong) UIWindow *window;

2.添加代码   

 @synthesize window = _window;