How to get parameter from dynamic route in route subscriber

I am trying to restrict access for non admin users on admin/structure/menu so that only admins can create any new links via admin/structure/menu.
Now I want that for one special menu with the ID "Focus" the "Add Link" button is available for another role too.

Here is a Screenshot of the actual backend with the "Add Link" still availabe. I want this to be gone for Hauptnavigation but not for Fokusnavigation.

Same thing when you click on a Menu. You get an "Add Link" button. I can disable this with me code below, but I don't know how to allow it for non Admin Roles again.

I can get the menu->id() in the my_menu.module file, with the same code like in the routesubscriber below but somehow it doesn't work in the routesubscriber. Drush CR returns an

PHP Fatal error:  Uncaught Error: Call to a member function id() on null in /var/www/html/web/modules/custom/my_menu/src/Routing/RouteSubscriber.php:31                                                                                                                
Stack trace:                                                                                                                                                                                                                                                           
#0 /var/www/html/web/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php(37): Drupal\my_menu\Routing\RouteSubscriber->alterRoutes(Object(Symfony\Component\Routing\RouteCollection))                                                                                  
#1 [internal function]: Drupal\Core\Routing\RouteSubscriberBase->onAlterRoutes(Object(Drupal\Core\Routing\RouteBuildEvent), 'routing.route_a...', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))                                              
#2 /var/www/html/web/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php(142): call_user_func(Array, Object(Drupal\Core\Routing\RouteBuildEvent), 'routing.route_a...', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#3 /var/www/html/web/core/lib/Drupal/Core/Routing/RouteBuilder.php(189): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object(Drupal\Core\Routing\RouteBuildEvent), 'routing.route_a...')                                                   
 in /var/www/html/web/modules/custom/my_menu/src/Routing/RouteSubscriber.php on line 31       

Are there other ways to get the value? What am I doing wrong.
Here is my code.

<?php

namespace Drupal\my_menu\Routing;

use Drupal\Core\Routing\RouteSubscriberBase;
use Drupal\Core\Routing\RoutingEvents;
use Symfony\Component\Routing\RouteCollection;
use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Class RouteSubscriber
 *
 * hiding menu editing components for non-admins
 *
 * @package Drupal\hw_menu\Routing
 */
class RouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  public function alterRoutes(RouteCollection $collection) {


    if ($route = $collection->get('entity.menu.add_link_form')) {

  //    dpm($route->getRequirements());

      $menu instanceof \Drupal\system\Entity\Menu;
      $menu = \Drupal::routeMatch()->getParameter('menu');
      if($menu->id() != 'focus') {
        $route->setRequirement('_role', 'administrator');
      }
    //  dpm($route->getRequirements());
    }

  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    // Run after content_translation, which has priority -210.
    $events[RoutingEvents::ALTER] = ['onAlterRoutes', -230];
    return $events;
  }

}

Would say that you code is failing because there is no menu entity available since the menu parameter converter works when the route gets resolved as commented by @Clive not during route build, which is when the alter event is fired.

On the other hand, why don't you use the Administer menus and menu links permission from core to deny your non-admin user's access to the menu administration? Also you can use Simple Menu Permissions or the Menu Admin per Menu modules if you want more control over your menus.

But if you still want to handle the menu access by yourself you can do that implementing your own access check using the route subscriber you have already.