Getting error while trying to send custom email with attachment In Magento 2.3.3

Fatal error: Uncaught Error: Call to a member function setBodyAttachment() on null in /home/vestuv/web/vestuviniaiziedai.lt/public_html/app/code/Dipolis/Callmeform/Mail/Template/TransportBuilder.php:11 Stack trace: #0 /home/vestuv/web/vestuviniaiziedai.lt/public_html/app/code/Dipolis/Callmeform/Controller/Index/Post.php(77):

Sent mail code

$fileDirectoryPath = $this->directoryList->getPath(MagentoFrameworkAppFilesystemDirectoryList::VAR_DIR).'/tmp';

                $filePath =  $fileDirectoryPath . '/' . $_FILES['attachment']['name'];
                move_uploaded_file($_FILES['attachment']['tmp_name'], $filePath);

                try {

                if($_FILES['attachment']['name']){

                    $fileName = $_FILES['attachment']['name'];
                    $mimeType = $_FILES['attachment']['type'];

                    $transport = $this->_transportBuilder->addAttachment(file_get_contents($filePath), $fileName, $mimeType);
                }

                $transport = $this->_transportBuilder                    
                    ->setTemplateIdentifier('callmeform_email_template')
                    ->setTemplateOptions(['area' => MagentoFrameworkAppArea::AREA_FRONTEND, 'store' => $storeId])
                    ->setTemplateVars([
                            'name'  => $username,
                            'phone'  => $userphone
                        ])
                    ->setFrom(['name' => $sentToName,'email' => $sentToEmail])
                    ->addTo($sentToEmail)
                    ->getTransport();

                $transport->sendMessage();

Attachment function

public function addAttachment($content, $fileName, $fileType)
    {
        $this->message->setBodyAttachment($content, $fileName, $fileType);

        return $this;
    }

public function setBodyAttachment($content, $fileName, $fileType)
    {
        $attachmentPart = $this->partFactory->create();
        $attachmentPart->setContent($content)
            ->setType($fileType)
            ->setFileName($fileName)
            ->setDisposition(Mime::DISPOSITION_ATTACHMENT)
            ->setEncoding(Mime::ENCODING_BASE64);
        $this->parts[] = $attachmentPart;
        return $this;
    }