Issue #3482559: Error in JsonApiFacetsDeriver::getDerivativeDefinitions() if serverInstance is NULL

Closes #3482559

modules/jsonapi_search_api_facets/src/Plugin/facets/facet_source/JsonApiFacetsDeriver.php

@@ -4,6 +4,7 @@ namespace Drupal\jsonapi_search_api_facets\Plugin\facets\facet_source;
 
 use Drupal\Component\Plugin\PluginBase;
 use Drupal\facets\FacetSource\FacetSourceDeriverBase;
+use Drupal\search_api\ServerInterface;
 
 /**
  * Derives a facet source plugin definition for every index.
@@ -17,7 +18,8 @@ class JsonApiFacetsDeriver extends FacetSourceDeriverBase {
     $derivatives = [];
     foreach ($this->entityTypeManager->getStorage('search_api_index')->getQuery()->execute() as $index) {
       // Only derive for an index that supports facets.
-      if ($this->entityTypeManager->getStorage('search_api_index')->load($index)->getServerInstance()->supportsFeature('search_api_facets')) {
+      $server = $this->entityTypeManager->getStorage('search_api_index')->load($index)->getServerInstance();
+      if ($server instanceof ServerInterface && $server->supportsFeature('search_api_facets')) {
         $derivatives[$index] = [
           'id' => $base_plugin_definition['id'] . PluginBase::DERIVATIVE_SEPARATOR . $index,
           'display_id' => strtr('jsonapi_search_api_facets_!index', ['!index' => $index]),