Quick Fix:

- On ticket edit, only display inactive help topic warning if the ticket has a help topic

include/staff/ticket-edit.inc.php

@@ -94,8 +94,7 @@ if ($_POST)
                     <option value="" selected >&mdash; <?php echo __('Select Help Topic');?> &mdash;</option>
                     <?php
                     if($topics=Topic::getHelpTopics()) {
-                      if(!array_key_exists($ticket->topic_id, $topics))
-                      {
+                      if($ticket->topic_id && !array_key_exists($ticket->topic_id, $topics)) {
                         $topics[$ticket->topic_id] = $ticket->topic;
                         $warn = sprintf(__('%s selected must be active'), __('Help Topic'));
                       }