iOS app working on iPhone(12.4.5) but not on iPad (13.3.1)

Question

Here is my problem: I have an iOS app under development. It runs fine on my iPhone 6 with iOS 12.4.5. When I want to run it on an iPad Mini with iOS 13.3.1; I get this in the debugging console of Xcode (Version 11.3.1):

2020-03-02 ... *** Terminating app due to uncaught exception 'NSInvalidArgumentException',
reason: 'Could not find a storyboard named 'Main' in bundle NSBundle
</private/var/containers/Bundle/Application/5...1/MyApp.app> (loaded)'
*** First throw call stack:
(0x1824e8a48 0x18220ffa4 ..... 0x1822ea360)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 

Does is make some sense to anyone?

It says "Could not find a storyboard ....". Indeed I have removed the storyboard, because I don't want it and this is no problem on my iPhone. Do I need to check (or do) something more to make it work on the iPad?

For information, here is the relevant code in the AppDelegate, in case it may be useful.

class AppDelegate: UIResponder,UIApplicationDelegate,   UNUserNotificationCenterDelegate,CLLocationManagerDelegate {
    var window: UIWindow?


    func application(_ application: UIApplication, 
        didFinishLaunchingWithOptions launchOptions:
        [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        window = UIWindow(frame: UIScreen.main.bounds)
        let navigatCtrlr = UINavigationController(), topVC = ViewController()
        navigatCtrlr.viewControllers = [topVC]
        navigatCtrlr.navigationBar.tintColor = UIColor.darkGray
        window!.rootViewController = navigatCtrlr
        window!.makeKeyAndVisible()
        .....

        return true
    }
    .....
}

Beside, the Main storyboard has been removed from the Info.plist