Hi Karl,
Change line 135 for:
$content .= '<span class="taskCompletedOnBy">(' .lang('completed on by', format_date($task->getCompletedOn()), $task->getCompletedBy() instanceof User ? $task->getCompletedBy()->getCardUrl() : '#', $task->getCompletedBy() instanceof User ? clean($task->getCompletedBy()->getDisplayName()) : lang('n/a')) . ')</span>
Basically, what this change does is first checking if the task is completed by a user and if it is then it gets the user's info. We have now scheduled an app-wide test searching for this kind of bug, trying to access a deleted user.
Thanks.