After upgrading to SS 2.4, I'm trying to run the nested URL migration task as described here: http://doc.silverstripe.org/sitetree#nested_hierarchical_urls

However, I am getting the following notice/trace and the process fails to do its job:

[Notice] Trying to get property of non-object
GET /dev/tasks/MigrateSiteTreeLinkingTask

Line 40 in /Users/Me/Sites/thesite/sapphire/tasks/MigrateSiteTreeLinkingTask.php

Source

31             'SELECT "ChildID", "FieldName" FROM "SiteTree_LinkTracking" WHERE "SiteTreeID" = %d',
32             $page->ID
33          ));
34          
35          foreach($tracking as $link) {
36             $linked = DataObject::get_by_id('SiteTree', $link['ChildID']);
37             
38             // TOOD: Replace in all HTMLText fields
39             $page->Content = preg_replace (
40                "/href *= *([\"']?){$linked->URLSegment}\/?/i",
41                "href=$1[sitetree_link id={$linked->ID}]",
42                $page->Content,
43                -1,
44                $replaced
45             );
46             
Trace

MigrateSiteTreeLinkingTask->run(SS_HTTPRequest)
Line 57 of TaskRunner.php
TaskRunner->runTask(SS_HTTPRequest)
Line 134 of RequestHandler.php
RequestHandler->handleRequest(SS_HTTPRequest)
Line 147 of Controller.php
Controller->handleRequest(SS_HTTPRequest)
Line 152 of RequestHandler.php
RequestHandler->handleRequest(SS_HTTPRequest)
Line 147 of Controller.php
Controller->handleRequest(SS_HTTPRequest)
Line 283 of Director.php
Director::handleRequest(SS_HTTPRequest,Session)
Line 127 of Director.php
Director::direct(/dev/tasks/MigrateSiteTreeLinkingTask)
Line 127 of main.php

Has anyone had a similar experience that could point me in the right direction?

Thanks for your help. I tried your suggestion and it worked for some links but not all. Upon running the link migration task a second time, it added some duplicate link code (I backed up of course, so no big deal).

Another related question - how are 301 redirects handled from old URLS? Or are they?