Drupal 8 - ajax callback & alter form

AjaxForm.php

<?php

class SimpleForm extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'simple_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['referentiel'] = [
      '#type' => 'select',
      '#title' => $this->t('Référentiel'),
      '#options' => ['foot' => $this->t('foot'), 'rugby' => $this->t('rugby'), 'basket' => $this->t('basket')],
      '#size' => 1,
    ];
    $form['titre'] = [
      '#type' => 'textfield',
      '#title' => $this->t('titre'),
      '#maxlength' => 64,
      '#size' => 64,
      '#default_value' => 'my title'
    ];
    $form['objet'] = [
      '#type' => 'textfield',
      '#title' => $this->t('objet'),
      '#maxlength' => 64,
      '#size' => 64,
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Display result.
    foreach ($form_state->getValues() as $key => $value) {
      drupal_set_message($key . ': ' . $value);
    }

  }
}

mymodule.php

<?php

// example.module
/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_simple_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  $form['#prefix'] = '<div id="my">';
  $form['#suffix'] = '</div>';

  $form['referentiel']['#ajax'] = array(
    'callback' => 'myCallback',
    'event' => 'change',
    'wrapper'  => 'my',
  );
}

function myCallback(array &$form, FormStateInterface $form_state) {
  $method = $form_state->getValue('referentiel');
  $form['titre']['#value'] = $method;
  $form['objet']['#value'] = $method;
  return $form;
}