Fix behavior of the SearchApiFulltext filter when no valid fulltext fields are configured

Closes #3511293

src/Plugin/views/filter/SearchApiFulltext.php

@@ -397,13 +397,15 @@ class SearchApiFulltext extends FilterPluginBase {
     if ($this->value === '') {
       return;
     }
-    $fields = $this->options['fields'];
-    $fields = $fields ?: array_keys($this->getFulltextFields());
+    $query = $this->getQuery();
+
+    $indexed_fulltext_fields = $query->getIndex()->getFulltextFields();
+    $fields = array_intersect($this->options['fields'], $indexed_fulltext_fields);
+    $fields = $fields ?: $indexed_fulltext_fields;
     // Override the search fields, if exposed.
     if (!empty($this->searchedFields)) {
       $fields = array_intersect($fields, $this->searchedFields);
     }
-    $query = $this->getQuery();
 
     // Save any keywords that were already set.
     $old = $query->getKeys();