The app delegate must implement the window property if it wants to use a main storyboard file swift
If you run your project on earlier than iOS 13.0, in that case you will face the problem. Because of iOS 13 and later, app launch differently than earlier versions.
In iOS 13 and later, use
UISceneDelegateobjects to respond to life-cycle events in a scene-based appIn iOS 12 and earlier, use the
UIApplicationDelegateobject to respond to life-cycle events.
When you launch the app in iOS 12 and earlier then UIApplicationMain class expect a window property in your AppDelegate class as like SceneDelegate has. So your problem will be solved if you add the following line in your AppDelegate class.
var window: UIWindow?For Objective-C
@property (strong, nonatomic) UIWindow *window;You can find more here App's Life Cycle.