使用未声明的类型＆＃39; WalkthroughViewController＆＃39;

Question

当我在我的框架源代码中使用下面的代码时，我收到以下错误：

  

使用未声明的类型＆＃39; WalkthroughViewController＆＃39;

override open func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    let defaults = UserDefaults.standard
    let hasViewedWalkthrough = defaults.bool(forKey: "hasViewedWalkthrough")

    if !hasViewedWalkthrough {
        print("succes")

        if let pageVC = storyboard?.instantiateViewController(withIdentifier: "WalkthroughViewController") as? WalkthroughViewController {
            present(pageVC, animated: true, completion: nil)
        }
    }
}

我该如何解决这个问题？