How can I export a PDF file using MYExportMenu in Yii?

Question

I'm trying to export a PDF file using MYExportMenu widget in Yii framework. This is currently what it looks like :

//...other MYExportMenu configuration
MYExportMenu::FORMAT_PDF => [
                            'pdfConfig' => [
                                'methods' => [
                                    'SetHeader' => $pdfHeader,
                                ]
                            ]
                        ],

But when I click on it, it starts a quick download-like popup, then it displays this error:

count(): Parameter must be an array or an object that implements Countable in C:\wamp64\www\myproject\vendor\mpdf\mpdf\classes\ttfontsuni.php

if (!$psName)
                throw new MpdfException("Could not find PostScript font name: " . $this->filename);
            // CHECK IF psName valid (PadaukBook contains illegal characters in Name ID 6 i.e. Postscript Name)
            $psNameInvalid = false;
            for ($i = 0; $i < count($psName); $i++) {
                $c = $psName[$i];
                $oc = ord($c);
                if ($oc > 126 || strpos(' [](){}<>/%', $c) !== false) {
                    //throw new MpdfException("psName=".$psName." contains invalid character ".$c." ie U+".ord(c));
                    $psNameInvalid = true;
                    break;

What I'm I missing?