func validateUserInput() {
  guard let userName = txtUserName.text else {
    show("UserName should not be empty.")
    return
  }
  guard let password = txtPassword.text else {
    show("Password should not be empty.")
    return
  }
  guard let accountNo = txtAccountNo.text else {
    show("Account Number should not be empty.")
    return
  }
  validateAccountDetails(userName, password, accountNo)
}