The AJAX callback for AutoTextfields returns the wrong form element

Problem/Motivation

AutoTextfields::textfieldsCallback() contains the following code.

  return $form['textfields_container'];

That is the wrong form element: AutoTextfields::buildForm() does not set $form['textfields_container'], but $form['text_fields_container'].

    // Wrap text fields in a container. This container will be replaced through
    // AJAX.
    $form['text_fields_container'] = [
      '#type' => 'container',
      '#attributes' => ['id' => 'text-fields-container'],
    ];
    $form['text_fields_container']['text_fields'] = [
      '#type' => 'fieldset',
      '#title' => $this->t("Generated text fields for first and last name"),
      '#description' => $this->t('This is where we put automatically generated text fields'),
    ];

Proposed resolution

Replace AutoTextfields::textfieldsCallback() with the following code.

  /**
   * Callback for ajax_example_auto_text_fields.
   *
   * Selects the piece of the form we want to use as replacement markup and
   * returns it as a form (renderable array).
   */
  public function textfieldsCallback($form, FormStateInterface $form_state) {
    return $form['text_fields_container'];
  }