Backend : correctif méthod Document::isValidClass()

common/models/Document.php

@@ -247,7 +247,7 @@ class Document extends ActiveRecordCommon
         return '';
     }
 
-    public function isValidClass($typeDocument)
+    public static function isValidClass($typeDocument)
     {
         return in_array($typeDocument, ['Invoice', 'DeliveryNote', 'Quotation']);
     }