Ich mache eine URLSession, aber für die URL sind Anmeldeinformationen erforderlich.

Ich habe diese ganze Methode hier, die versucht, eine URLSession mit URLCredentials durchzuführen:

func loginUser(_ username: String, password: String, completion: @escaping (_ result: Bool) -> Void)
    {
        //Create request URL as String

        let requestString = String(format:"%@", webservice) as String

        //Covert URL request string to URL

        guard let url = URL(string: requestString) else {
            print("Error: cannot create URL")
            return
        }

        //Convert URL to URLRequest

        let urlRequest = URLRequest(url: url)

        print(urlRequest)

        //Add the username and password to URLCredential

        credential = URLCredential(user:username, password:password, persistence: .forSession)

        //Setup the URLSessionConfiguration

        let config = URLSessionConfiguration.default

        //Setup the URLSession

        let session = URLSession(configuration: config, delegate: self, delegateQueue: nil)

        //Prepare the task to get data.

        let task = session.dataTask(with: urlRequest, completionHandler: { (data, response, error) in

            DispatchQueue.main.async(execute: {

                if(error == nil)
                {
                    completion(true)
                }
                else
                {
                    completion(false)
                }

            })

        })

        //Run the task to get data.

        task.resume()

    }

und hier sind meine URLSessionDelegate-Methoden:

func urlSession(_ session: URLSession, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {

        if challenge.previousFailureCount > 0
        {
            completionHandler(Foundation.URLSession.AuthChallengeDisposition.cancelAuthenticationChallenge, nil)
        }
        else
        {
            completionHandler(Foundation.URLSession.AuthChallengeDisposition.useCredential, URLCredential(trust:challenge.protectionSpace.serverTrust!))
        }

    }

    /**
     Requests credentials from the delegate in response to an authentication request from the remote server.
     */

    func urlSession(_ session: URLSession, task: URLSessionTask, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {

        completionHandler(Foundation.URLSession.AuthChallengeDisposition.useCredential,credential)

    }

Ich bemerke, wenn ich das in dieser Delegate-Methode debugge:

func urlSession(_ session: URLSession, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {

        if challenge.previousFailureCount > 0
        {
            completionHandler(Foundation.URLSession.AuthChallengeDisposition.cancelAuthenticationChallenge, nil)
        }
        else
        {
            completionHandler(Foundation.URLSession.AuthChallengeDisposition.useCredential, URLCredential(trust:challenge.protectionSpace.serverTrust!))
        }

    }

Diese Methode wird zweimal aufgerufen, und wenn sie zum zweiten Mal auf diese Zeile trifft:

completionHandler(Foundation.URLSession.AuthChallengeDisposition.useCredential, URLCredential(trust:challenge.protectionSpace.serverTrust!))

Ich erhalte diesen Fehler:

fatal error: unexpectedly found nil while unwrapping an Optional value

und dann stürzt meine App ab! Wie behebe ich diesen Fehler?