AppDelegate未显示情节提要按钮

我正在设置一个mainviewcontroller,通过以下方式保存多个viewcontroller:

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

        let pageController = UIPageViewController(
            transitionStyle: UIPageViewControllerTransitionStyle.scroll,
            navigationOrientation: UIPageViewControllerNavigationOrientation.horizontal,
            options: nil
        )

        let navigationController = MainViewController(rootViewController: pageController)
        navigationController.view.backgroundColor = UIColor.white

        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let leftView = storyboard.instantiateViewController(withIdentifier: "VC1")
        let middleView = storyboard.instantiateViewController(withIdentifier: "VC2")
        let rightView = storyboard.instantiateViewController(withIdentifier: "VC3")

        navigationController.viewControllerArray = [leftView, middleView, rightView]

        self.window!.rootViewController = navigationController
        self.window!.makeKeyAndVisible()

        return true
    }
}

我的mainviewcontroller还有一个浮动按钮。我已经在故事板中熟悉了这个(见下图)。问题是按钮没有显示。我怎样才能让它显示出来? 我知道这个问题与我在appdelegate中添加mainviewcontroller有关,但我不确定这是如何隐藏按钮的。