Fix(Core): fix undefined index

Author: stonebuzz

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 
+- Fix undefined index
 - Prevent business rules from triggering twice on tickets
 
 ## [2.9.19] - 2026-27-01

inc/ticket.class.php

@@ -864,7 +864,11 @@ public static function linkedTickets(CommonDBTM $ticket, $status = CommonITILObj
         $linkedTicket = new Ticket();
 
         foreach ($tickets as $data) {
-            if ($data['link'] !== Ticket_Ticket::LINK_TO || !$linkedTicket->can($data['tickets_id'], UPDATE)) {
+            $ticket_id_missing = empty($data['tickets_id']);
+            $link_invalid = empty($data['link']) || $data['link'] !== Ticket_Ticket::LINK_TO;
+            $no_permission = !$linkedTicket->can($data['tickets_id'], UPDATE);
+
+            if ($ticket_id_missing || $link_invalid || $no_permission) {
                 continue;
             }