Fix failing test

Author: SOHELAHMED7

src/lib/AttributeResolver.php

@@ -434,7 +434,7 @@ protected function prepareIndexes(array $indexes):array
                 $props = $index;
                 $indexType = null;
             }
-            if (strtolower($indexType) === 'unique') {
+            if (strtolower((string) $indexType) === 'unique') {
                 $indexType = null;
                 $unique = true;
             }