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