Newbie to a great component.....problem with Views

Just installed this componet....putting it into test on my website.....but got this error....can you please let me know how to fix this....

Fatal error: Call to a member function setQuery() on a non-object in /content/HostingPlus/w/f/wfc.ie/web/components/com_pmform/views/complete/view.html.php on line 28


Code line is as follows :

class PmFormViewComplete extends JView
{
function display($tpl = null)
{
$config = PmFormHelper::getConfig() ;
$id = JRequest::getInt('id', 0) ;
$sql = 'SELECT thanks_message FROM #__pf_forms AS a INNER JOIN #__pf_payments AS b ON a.id = b.form_id WHERE b.id='.$id;
$db->setQuery($sql) ; // this is the error line 28
$thanksMessage = $db->loadResult();
if ($thanksMessage)
$config->thanks_message = $thanksMessage ;
$message = $config->thanks_message;
$offline = JRequest::getInt('offline_payment', 0) ;
if ($offline) {
$message = $config->thanks_message_offline ;
}
$message = nl2br($message);
$this->assignRef('message', $message);
parent::display($tpl);
}
}