Load WKWebViewController then you can't transition to it again

Question

I have the following view controller

import Foundation
import UIKit
import WebKit

class TextViewController : UIViewController, WKNavigationDelegate {

    var data: Book!

    @IBOutlet weak var webView: WKWebView!

    override func viewDidLoad() {
        super.viewDidLoad()
        self.navigationController?.navigationBar.isHidden = true
        webView.navigationDelegate = self

        if var body = self.data.body {
            body += "<meta name=\"viewport\" content=\"initial-scale=1.0\" />"
            webView.loadHTMLString(body, baseURL: nil)
        }
    }

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

        NetworkActivityIndicatorManager.networkOperationStarted()
        LoadingIndicatorView.show("Loading")


        self.tabBarController?.tabBar.isHidden = true;
        self.navigationController?.navigationBar.isHidden = true
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated);
        self.tabBarController?.tabBar.isHidden = false;
    }

    @IBAction func goback(_ sender: UIButton) {
        self.navigationController?.popViewController(animated: true)
    }

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        let css = "body { font-size: 18px; line-height: 28px; } h1 { font-size: 34px; text-align: left; margin-bottom: 12px; line-height: 1.3;  } h2 { font-size: 26px; font-weight: 700; padding: 0; margin-bottom: 10px; text-align: left; line-height: 34.5px; letter-spacing: -0.45px; } p, i, a { margin-top: 21px; font-size: 20px; letter-spacing: -0.03px; line-height: 1.57; }";

        let js = "var style = document.createElement('style'); style.innerHTML = '\(css)'; document.head.appendChild(style);"

        webView.evaluateJavaScript(js, completionHandler: nil)


        NetworkActivityIndicatorManager.networkOperationFinished()
        LoadingIndicatorView.hide()
    }

}

When I transition to this view controller, it loads the webview. However, when I exit the view controller by triggering the goback action, I am unable to go back to the webview controller. I'm I missing something. Here's how I segue to the controller -

self.performSegue(withIdentifier: "toText", sender: nil)