bugfix: insertBefore requires null and not undefined

js/erp5_loader.js

@@ -4541,7 +4541,7 @@
             {"text": title }
           );
           if (merger) {
-            row.insertBefore(cell, (set === undefined ? undefined : row.childNodes[check ? 1 : 0]) );
+            row.insertBefore(cell, (set === undefined ? null : row.childNodes[check ? 1 : 0]) );
             set = true;
           } else {
             row.appendChild(cell);