Drupal 8 - ajax callback

AjaxForm.php

<?php
class AjaxForm extends FormBase {


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

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

    $form['#tree'] = TRUE;
    $form_state->setCached(FALSE);
    $form['container'] = [
      '#type' => 'container',
      '#attributes' => ['id' => 'my-container'], // CHECK THIS ID
    ];

    $form['container']['selection'] = [
      '#type' => 'select',
      '#title' => t('Selection'),
      '#options' => [
        'selection1' => 'Selection 1',
        'selection2' => 'Selection 2',
        'selection3' => 'Selection 3',
      ],
      '#empty_value' => '',
      '#ajax' => [
        'callback' => [$this, 'myCallback'],
        'event' => 'change',
        'wrapper' => 'my-container',
      ],
    ];

    if ($form_state->getUserInput()) {
      $val = 'changed';
      $status = FALSE;
    }
    else {
      $val = 'original';
      $status = TRUE;
    }

    $form['container']['label'] = [
      '#type' => 'textfield',
      '#title' => 'label',
      '#attributes' => [
        'id' => ['edit-label'],
      ],
      '#value' => $val,
    ];

    $form['container']['title'] = [
      '#type' => 'textfield',
      '#title' => 'title',
      '#attributes' => [
        'id' => ['edit-title'],
      ],
      '#disabled' => $status,
    ];


    // Add a submit button that handles the submission of the form.
    $form['actions'] = [
      '#type' => 'actions',
      'submit' => [
        '#type' => 'submit',
        '#value' => $this->t('Submit'),
      ],
    ];

    return $form;
  }


  function myCallback(array &$form, FormStateInterface $form_state) {
        return $form['container'];
    }

    /**
   * {@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);
    }
  }
}