Great catch! I'll fix this for the (inevitable) version 1.2.1 release.
In the meantime you can fix it by changing this line in qa-app-format.php:
$fields['who_2']=qa_who_to_html($post['lastuserid']==$userid, $post['lastuserid'], $usershtml, null, false);
... to ...
$fields['who_2']=qa_who_to_html(isset($userid) && ($post['lastuserid']==$userid), $post['lastuserid'], $usershtml, null, false);