protect from invalid array access in RecursiveFormatterHelper

Author: Nil Portugues Caldero

src/Transformer/Helpers/RecursiveFormatterHelper.php

@@ -42,8 +42,10 @@ public static function getIdPropertyAndValues(array &$mappings, array &$value, $
         $idProperties = self::getIdProperties($mappings, $type);
 
         foreach ($idProperties as &$propertyName) {
-            $values[] = self::getIdValue($value[$propertyName]);
-            $propertyName = \sprintf('{%s}', $propertyName);
+            if (array_key_exists($propertyName, $value)) {
+                $values[] = self::getIdValue($value[$propertyName]);
+                $propertyName = \sprintf('{%s}', $propertyName);
+            }
         }
         self::flattenObjectsWithSingleKeyScalars($values);