diff --git a/src/EventListener/DataContainer/SaveClasses.php b/src/EventListener/DataContainer/SaveClasses.php
index 51d21c2..d400b5c 100644
--- a/src/EventListener/DataContainer/SaveClasses.php
+++ b/src/EventListener/DataContainer/SaveClasses.php
@@ -77,6 +77,6 @@ public function onLoadCallback($value, $dc): array
 
         $options = array_unique(array_merge(...$options));
 
-        return array_values(array_filter($options, static fn ($option) => 1 === preg_match(sprintf('/(^|\s)(%s)(\s|$)/', preg_quote($option)), $dc->activeRecord->toolbox_classes)));
+        return array_values(array_filter($options, static fn ($option) => 1 === preg_match(sprintf('/(^|\s)(%s)(\s|$)/', preg_quote($option)), $dc->activeRecord->toolbox_classes ?? '')));
     }
 }