添加UIBarButtonItem无法触发

Question

语言：swift 3.0

os：macOS 10.12.3

xcode：8.2.1

这是一个带有ViewController的空白Storyboard。

以下是代码：

import UIKit
import WebKit

class CommonWebViewViewController: UIViewController, WKUIDelegate, WKNavigationDelegate {

    let backButton = UIBarButtonItem(title: "BACK", style: .plain, target: self, action: #selector(goBack))
    let closeButton = UIBarButtonItem(title: "CLOSE", style: .plain, target: nil, action: #selector(close))

    let webview = WKWebView()
    var url = ""

    override func viewDidLoad() {
        super.viewDidLoad()
        view = webview

        navigationItem.setLeftBarButtonItems([backButton], animated: true)

        webview.uiDelegate = self
        webview.navigationDelegate = self
        webview.allowsBackForwardNavigationGestures = true
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        title = webView.title
    }

    func load(withUrl url: String) {
        self.url = url
        webview.load(URLRequest(url: URL(string: self.url)!))
    }

    func goBack() {
        if webview.canGoBack {
            webview.goBack()
            navigationItem.leftBarButtonItems?.append(closeButton)
        }
        else {
            close()
        }
    }

    func close() {
        _ = self.navigationController?.popViewController(animated: true)
    }

}

我想制作一个浏览器，当我点按“返回”按钮时，除非我点击网页浏览中的链接，否则没有响应。

P.S如果我在viewDidload方法中创建UIBarButtonItem对象，问题就会解决。