When upgrading to drupal 9 I got the following errors in my EventSubscribers:

drupal 9 must be an instance of Symfony\Component\HttpKernel\Event\ResponseEvent, instance of Symfony\Component\HttpKernel\Event\RequestEvent given

Fix the issue by

changing your getSubscribedEvents()

  public static function getSubscribedEvents() {
    $events[KernelEvents::RESPONSE][] = array('checkAuthStatus');
    return $events;

On top of the file, use

use Symfony\Component\HttpKernel\Event\ResponseEvent;

Your response code change to:

  * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
  public function checkAuthStatus(ResponseEvent $event) {

This error occurs due to a symphony upgrade from 4.4 to 5.