throw error instead of returning null data

Avoids WSOD when EmitEvent creates a StompHeaderEvent

src/Plugin/Action/AbstractGenerateDerivativeMediaFile.php

@@ -40,7 +40,7 @@ class AbstractGenerateDerivativeMediaFile extends AbstractGenerateDerivativeBase
   protected function generateData(EntityInterface $entity) {
     $data = parent::generateData($entity);
     if (get_class($entity) != 'Drupal\media\Entity\Media') {
-      return;
+      throw new \RuntimeException("Entity {$entity->getEntityTypeId()} {$entity->id()} is not a media", 500);
     }
     $source_file = $this->mediaSource->getSourceFile($entity);
     if (!$source_file) {