php - DOMPDF not working, giving error

I have form.php file that getting data from mysql table.
form.php

 if(mysql_num_rows($sql)==0){
?>





    
        
1.
        
America
    

Export as PDF
 } else {
?>





    
        
1.
        
India
    

Export as PDF
 }
?>

And pdf.php

require_once("dompdf/dompdf_config.inc.php");
ob_start();
$html = file_get_contents('form.php');
$dompdf = new DOMPDF();
date_default_timezone_set('Asia/Kolkata');
$tym = date('g:i s');
$filename = 'FAA-8130_3_'.$tym;
$dompdf->load_html($html);
ob_end_flush();
$dompdf->render();
$dompdf->stream($filename. ".pdf", array("Attachment" => 0));

When I try to export it as PDF file, it is giving error...

Fatal error: Call to a member function prepend_child() on a non-object in C:\wamp\www\path\to\dompdf\include\frame_tree.cls.php on line 231

I could not understand why it is happening. I viewed this,this and this, but failed to resolve.