func loginUser(_ username: String, password: String, completion: @escaping (_ result: Bool) -> Void)
    {
        //Setup the NSURLSessionConfiguration

        let configuration = URLSessionConfiguration.default

        //Setup the NSURLSession

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

        //Add the username and password to NSURLCredential

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

        //Create request URL as String

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

        //Convert URL string to NSURL

        let url: URL! = URL(string: requestString)

        //Prepare the task to get data.

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

            DispatchQueue.main.async(execute: {

                if(error == nil)
                {

                    //If there is no error calling the API, return true

                    completion(true)
                }
                else
                {

                    //If there is an error calling the API, return false

                    completion(false)
                }

            })

        })

        //Run the task to get data.

        task.resume()

    }

fatal error: unexpectedly found nil while unwrapping an Optional value

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

            DispatchQueue.main.async(execute: {

                if(error == nil)
                {

                    //If there is no error calling the API, return true

                    completion(true)
                }
                else
                {

                    //If there is an error calling the API, return false

                    completion(false)
                }

            })

        })