refs #14719 - bugfixes getRID() method at SolrDefaultFincTrait

* bugfixes ID lookup
* now uses correct value retrieval method

module/finc/src/finc/RecordDriver/SolrDefaultFincTrait.php

@@ -857,7 +857,11 @@ trait SolrDefaultFincTrait
             && !empty($records)
         ) {
             foreach ($records as $record) {
-                $retval[$record->getRID()] = $record->getUniqueID();
+                // we SHOULD use the immediate field values here since the
+                // above query yields record stubs of type VuFind\RecordDriver\SolrDefault
+                // which normally not allow for specialized functions e.g. SolrMarcFinc::getRID
+                // the specified fields, however, will always be set
+                $retval[$record->fields['record_id']] = $record->fields['id'];
             }
         }