Fields using the entity reference views handler can cause a RuntimeException when normalized.

#2940339: Port reference field support for non-empty entity reference fields not pointing to an entity from #2543726 added this:

    if ($target_entity === NULL) {
      $host_entity = $parent->getHostEntity();
      $relatable_resource_types = $resource_type_repository->get(
        $host_entity->getEntityTypeId(),
        $host_entity->bundle()
      )->getRelatableResourceTypes()[$parent->getPropertyName()];
      if (count($relatable_resource_types) !== 1) {
        throw new \RuntimeException('Relationships to virtual resources are possible only if a single resource type is relatable.');
      }

In certain circumstances, entity reference fields using the views reference handler causes $relatable_resource_types to be set as an array containing every bundle of the given entity type, causing the above RuntimeException to be thrown.

This has occurred in a case where the view in use by the handler did have a condition limiting the options to a single bundle.

Interestingly, the exception does not get thrown in all cases. So far, I've only seen it when normalizing a single entity for which the given field is empty. Selecting a value in the is field fixes the issue and it remains fixed after unsetting the value and saving the entity again.

For now I have worked around the issue by implementing hook_entity_bundle_field_info_alter() and setting the target_bundles handler setting for the given field / entity.

Interestingly, when the field does have a value, it is represented in the normalized output with a uuid for it's id, i.e. it does not appear to be a relationship to "virtual" resources.