Error loading a web in a webview in swift

I have a webview in which I pass a url, it usually works fine, but there are other times that I get this error: libc++abi.dylib: terminating with uncaught exception of type NSException

The problem is that I do not know where the error is.

I added a try-catch , but it does not solve anything

let url:URL? = URL(string: self.url!)
var urlRequest: URLRequest?
if let actualUrl = url {
    do {
        urlRequest = try URLRequest(url: actualUrl)
    } catch {
        fatalError("Unhandled error: \(error)")
    }
    self.webView.loadRequest(urlRequest!)
}

The error appears once you have loaded the web

How can I know where the error is?

The complete code:

import UIKit

class WebViewViewController: UIViewController, UIWebViewDelegate {

    var url: String?

    let progressAlert: UIAlertController = UIAlertController(title: "Cargando Contenido", message: "Espere, por favor", preferredStyle: UIAlertControllerStyle.alert)
    let loadingIndicator: UIActivityIndicatorView = UIActivityIndicatorView()

    @IBOutlet weak var webView: UIWebView!
    @IBOutlet weak var btnVolver: UIBarButtonItem!

    override func viewDidLoad() {
        super.viewDidLoad()

        print("webviewcontroller")

        let attributes = [NSFontAttributeName: UIFont.fontAwesomeOfSize(20)] as Dictionary!
        btnVolver.setTitleTextAttributes(attributes, for: .normal)
        btnVolver.title = String.fontAwesomeIconWithName(.ArrowLeft)
        btnVolver.tintColor = UIColor.white
        //btnVolver.action = #selector(volver)
        btnVolver.target = self
        self.navigationItem.setHidesBackButton(true, animated: true)
        self.navigationItem.leftBarButtonItem = btnVolver

        webView.delegate = self
        self.navigationController!.navigationBar.isHidden = false
    }

    @IBAction func volver(_ sender: UIBarButtonItem) {
        self.performSegue(withIdentifier: "volver", sender: nil)
    }

    func webViewDidStartLoad(_ webView: UIWebView) {
        loadingIndicator.center = CGPoint(x: progressAlert.view.bounds.size.width/2, y: progressAlert.view.bounds.size.height - 10)
        loadingIndicator.hidesWhenStopped = true
        loadingIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.gray
        loadingIndicator.startAnimating()

        progressAlert.view.addSubview(loadingIndicator)
        self.present(progressAlert, animated: true, completion: nil)
        //progressAlert.present(self, animated: true, completion: nil)
    }

    func webViewDidFinishLoad(_ webView: UIWebView) {
        progressAlert.dismiss(animated: true, completion: nil)
    }

    override func viewDidAppear(_ animated: Bool) {
        if let actualArticle = self.articleToDisplay {
            let url = NSURL(string: actualArticle.enlace)
            if let actualUrl = url {
                let urlRequest = NSURLRequest(url: actualUrl as URL)
                self.webView.loadRequest(urlRequest as URLRequest)
            }
        }
        else if let actualArticle = self.tweetToDisplay {
            let url = NSURL(string: actualArticle.enlace)
            if let actualUrl = url {
                let urlRequest = NSURLRequest(url: actualUrl as URL)
                self.webView.loadRequest(urlRequest as URLRequest)
            }
        }
        else if let enlace = self.url {
            let url = NSURL(string: enlace)
            if let actualUrl = url {
                let urlRequest = NSURLRequest(url: actualUrl as URL)
                self.webView.loadRequest(urlRequest as URLRequest)
            }
        }
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}