I have an universal app I am writing and I initially wrote it using two storyboard files.  I had code in the App Delegate didFinishLaunchingWithOptions routine to decide which storyboard to load and away it would go.  I have since realised that that was a silly thing to do because I had duplicate code, so I have deleted one storyboard and made the other universal.  I have fixed up the VCs to point at the right class and everything.  However, my app now refuses to launch.  When I run it in the sim, it gives me the error 
The app delegate must implement the window property if it wants to use a main storyboard file.
.
Here is my code in App Delegate:
func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    // Override point for customization after application launch.
    var deviceIdiom = UIDevice.currentDevice().userInterfaceIdiom
    if deviceIdiom == UIUserInterfaceIdiom.Phone {
        strDevice = "iPhone"
    } else if deviceIdiom == UIUserInterfaceIdiom.Pad {
        strDevice = "iPad"
    } else {
        strDevice = "Unknown"
    }
    return true
}
What am I doing wrong?
(Questions I've already looked at but didn't help):
Unique Issue displaying first storyboard scene in Xcode
The app delegate must implement the window property if it wants to use a main storyboard file
Managing two storyboards in App Delegate
How to manually set which storyboard view to show in app delegate
Swift - Load storyboard programatically