Delete hook throws Drupal\Core\Entity\EntityStorageException: Update existing 'node' entity while changing the ID is not supported.

In my case, I have a site with users containing CER fields, meaning I can't delete users while the that CER is enabled. My workaround is to temporarily disable the CER, delete the users, then re-enable the CER.

This bug happens when other delete hooks run while cer_entity_delete is in flight, for instance when deleting certain referenced items on hook_entity_delete that have a cer controlled field.

This causes \Drupal\cer\Entity\CorrespondingReference::synchronizeCorrespondingFields() to run with a list of entities that are in the loader cache, but no longer exist. EntityStorageBase won't be able to load the original anymore, causing $entity->original to be empty, triggering the exception.

I found that calling the following and checking for non-null during the inner loop in \Drupal\cer\Entity\CorrespondingReference::synchronizeCorrespondingFields() fixes the issue for that particular site.

$original = $this->entityTypeManager()
              ->getStorage($correspondingEntity->getEntityTypeId())
              ->loadUnchanged($correspondingEntity->id());
if ($original) {
  $this->synchronizeCorrespondingField($entity, $correspondingEntity, $correspondingField, $operation);
}