func loginUser(from url: URL, with username: String, and password: String, completionHandler: @escaping (DrivetimeUserProfile?, DrivetimeAPIError.LoginError?) -> Void) {
        DispatchQueue.main.async {
            completionHandler(nil, .EmptyData)
        }
    }

@IBAction func onLoginButtonTapped(_ sender: Any) {

        guard let url = URL(string: USER_LOGIN_ENDPOINT) else { return }

        let username = userNameTextField.text
        let password = passwordTextField.text

        client.loginUser(from: url, with: username!, and: password!) { (data, error) in

            if let error = error {

                switch error {

                case .EmptyData:
                    DispatchQueue.main.async {
                        presentAlertVC()
                    }

                case .CannotDecodeJson:
                    DispatchQueue.main.async {
                         presentAlertVC()
                    }
                }
            }
        }

func testLoginButtin_ShouldPresentAlertContollerIfErrorIsNotNil() {

    sut.onLoginButtonTapped(sut.loginButton)

    let alert = sut.presentingViewController

    XCTAssertNotNil(alert)


}