check for null entry textbox xamarin

gistfile1.txt

 private void btnSign_touchUp(object sender, EventArgs e)
        {
            if (!IsValid)
            {
                var okAlertController = UIAlertController.Create("Alert", _validationError, UIAlertControllerStyle.Alert);
                okAlertController.AddAction(UIAlertAction.Create("OK", UIAlertActionStyle.Default, (action) => { }));
                UIApplication.SharedApplication.KeyWindow.RootViewController.PresentViewController(okAlertController, true, null);
                return;
            }


//Check that the email and password are not empty

        private bool IsValid
        {
            get
            {
                var isValid = false;
                if (String.IsNullOrWhiteSpace(txtUserName.Text) || String.IsNullOrWhiteSpace(txtPassword.Text)  )
                {
                    _validationError = "Email or Username and Password is required.";
                }
               else
                {
                    _validationError = String.Empty;
                    isValid = true;
                }
                return isValid;
            }
        }