MPDF7 not displaying PDF

Question

My MPDF is not properly displaying PDF. I'm currently developing in Cakephp 2.x and I encountered an error on MPDF 7 when I tried to install it. I've already updated my composer dependencies but it still didn't work.

My view/layout:

 <?php 
    require_once(APP . 'Vendor' . DS . 'autoload.php');

    $mpdf = new \Mpdf\Mpdf();
    $mpdf->WriteHTML($this->fetch('content'));
    $mpdf->Output();
 ?>

Attached here is the image of the generated error.

MPDF not displaying PDF