Cannot display draft nor scheduled mailings under Joomla- error "Using $this when not in object context"

This works under 4.7.16, but under 4.7.20 we get this error under Joomla.  The Drupal demo site is OK, but it fails under two Joomla instances. As the Joomla demo site has gone (and was out of date anyway) I can't test it there.  I've isolated where the error occurs: [cms.root]\administrator\components\com_civicrm\civicrm\CRM\Utils\PagerAToZ.php lines 149 to 155:

    $qfKey = NULL;
    if (isset($query->_formValues)) {
      $qfKey = CRM_Utils_Array::value('qfKey', $query->_formValues);
{{    }}}
    if (empty($qfKey)) {
      $qfKey = CRM_Utils_Request::retrieve('qfKey', 'String', $this, FALSE, NULL, $_REQUEST);
{{    }}}

$this is not when the createLinks function is called.  I'm not sure how we go from there.