MPDF致命错误：在null上调用成员函数WriteHTML（）[关闭]

I want export data into pdf file so I try to use mpdf. I am following step by step of its tutorial, But I am getting error

Fatal error: Call to a member function WriteHTML() on null in C:\xampp\htdocs\laboratorium\application\controllers\beranda.php on line 506

In controller

$this->load->library('M_pdf');
    $this->load->model('mod');
    $data['result'] = $this->mod->getReport();

    $html = $this->load->view('report', $data);
    $this->M_pdf->pdf->WriteHTML($html);

    //download it.
    $this->m_pdf->pdf->Output($pdfFilePath, "D");   

And I am sure that $html is not null. I try to load it and it is working. Why this is happen?

try this

   $this->load->library('M_pdf');
    $this->load->model('mod');
    $data['result'] = $this->mod->getReport();

    $html = $this->load->view('report', $data,true);
    $this->m_pdf->pdf->WriteHTML($html);

    //download it.
    $this->m_pdf->pdf->Output($pdfFilePath, "D");