So at the end of the chapter, to handle activation events coming from Spotlight, we're told to place the following into our AppDelegate class:

func application(_ application: UIApplication, continue userActivity: NSUserActivity, restorationHandler: @escaping ([UIUserActivityRestoring]?) -> Void) -> Bool {
    if (userActivity.activityType == CSSearchableItemActionType) {
        if let uniqueIdentifier = userActivity.userInfo?[CSSearchableItemActivityIdentifier] as? String {
            if let navigationController = window?.rootViewController as? UINavigationController {
                if let viewController = navigationController.topViewController as? ViewController {
                    viewController.showTutorial(Int(uniqueIdentifier)!)
                }
            }
        }
    }

    return true
}

Even copying right out of the book, however, it indicates that window is an unresolved identifier, and shows a compiler error.

Is this a version/upgrade issue, or should there be a property added to AppDelegate to handle this?

That did it! Only other note, besides of course moving over the import CoreSpotlight call is to remove the return true call at the end of the function, since the new version is a void function.

Thanks!