Facet form broken for non-Search API facet sources

Thanks for the patch. But I wonder if this is the correct solution.
Neither the HierarchyProcessor nor the Hierarchy Plugin should be limited to Search API. So the problem might be the something else:

This is because the submit assumes a hierarchy plugin is always there, but it only gets set for Search API facets source

The question is, why isn't it set?

function facets_update_8008() {
  $config_factory = \Drupal::configFactory();

  foreach ($config_factory->listAll('facets.facet.') as $facet_config_name) {
    $facet = $config_factory->getEditable($facet_config_name);
    $facet->set('hierarchy', ['type' => 'taxonomy', 'config' => []]);
    $facet->save(TRUE);
  }
}

Previously the hierarchy was hardcoded to be taxonomy. The update hook above should set the plugin instead for all existing facets. So the question is under which circumstances the exception is thrown?

The taxonomy plugin is currently limited to the Search API backend. But as we now allow 3rd party hierarchy plugins we must not limit this functionality to Search API anymore.

Here's a quick and dirty patch. It would be better to introduce a feature to let plugins declare their backend compatibilty. But there's no list of well-known backends yet.