Issue #3209239 by longwave: [Symfony 5] Symfony\Contracts\EventDispatcher\Event subclasses need return types

core/lib/Drupal/Core/File/Event/FileUploadSanitizeNameEvent.php

@@ -118,7 +118,7 @@ class FileUploadSanitizeNameEvent extends Event {
    *
    * @see \Drupal\system\EventSubscriber\SecurityFileUploadEventSubscriber
    */
-  public function stopPropagation() {
+  public function stopPropagation(): void {
     throw new \RuntimeException('Propagation cannot be stopped for the FileUploadSanitizeNameEvent');
   }