¿Cómo crear ventanas modales en drupal 8? (Actualizado)

DefaultController.php

<?php
// fyi: path src/Controller/DefaultConotroller.php
/**
 * @file
 * Contains Drupal\mymodule\Controller\DefaultController.
 */
 
namespace Drupal\mymodule\Controller;
 
use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Ajax\AjaxResponse; 
use Drupal\Core\Ajax\OpenModalDialogCommand;
use Drupal\Core\Url;
use Drupal\Core\Link;
 
/**
 * Class DefaultController.
 *
 * @package Drupal\mymodule\Controller
 */
class DefaultController extends ControllerBase {
 
  /**
   * Index.
   *
   * @return string
   *   Return Hello string.
   */
  public function index() {
    $attributes = [
      'attributes' => [
        'class' => ['use-ajax'],
        'data-accepts' => 'application/vnd.drupal-dialog',
      ],
    ];
    // Let's create the link.
    $url = Url::fromRoute('mymodule.modal', [], $attributes);
    $internal_link = Link::fromTextAndUrl($this->t('Open Modal'), $url)->toString();
    return [
      '#type' => 'markup',
      '#markup' => $internal_link,
      '#attached' => ['library' => ['core/drupal.dialog.ajax']]
    ];
  }
   
  /**
   * Displays modal.
   */
  public function modal() {
    $response = new AjaxResponse();
    $title = $this->t('Title of Modal');
    $content['#markup'] = 'Content of Modal';
    $response->addCommand(new OpenModalDialogCommand($title, $content));
    return $response;
  }
}

mymodule.info

name: mymodule
type: module
description: My Awesome Module
core: 8.x
package: Custom

mymodule.routing.yml

mymodule.default_controller_index:
  path: '/mymodule/index'
  defaults:
    _controller: '\Drupal\mymodule\Controller\DefaultController::index'
    _title: 'mymodule Title'
  requirements:
    _permission: 'access content'
     
mymodule.modal:
  path: '/mymodule/modal'
  defaults:
    _controller: '\Drupal\mymodule\Controller\DefaultController::modal'
    _title: 'mymodule Title'
  requirements:
    _permission: 'access content'