update

app/Helpers/helpers.php

@@ -7,7 +7,7 @@ if (!function_exists('list_to_tree')) {
     {
         $branch = [];
         foreach ($elements as $element) {
-            if ($element['$parentKey'] == $parentId) {
+            if ($element[$parentKey] == $parentId) {
                 $children = list_to_tree($elements, $element['id']);
                 if ($children) {
                     $element['children'] = $children;