Grav CMS: Output message in the Debug Bar from native PHP

Question

I'm working on a custom Twig filter which resizes your images to a set width (and auto height).

{# TWIG CODE #}
{% set imgpath = page.header.custom.bgimage|first.path %}
<div class="thumb" style="background-image: url({{ url(imgpath|resize(240)) }})"></div>

It works great so far, but I encountered some errors when the ordering of the pages is changed. I'd like to use the Grav Debug Bar for debugging, since it's very convenient and keeps the code clean.

Inside Twig, you can simply use {{ dump(message) }}.
Unfortunately the resizing process happens inside native PHP, so I need a way to output messages from PHP to the Grav Debug Bar.
As stated inside the Docs, you can use $grav['debugger']->addMessage($this).

This throws an error when calling the resize Twig filter:

Twig_Error_Runtime
An exception has been thrown during the rendering of a template ("Undefined variable: grav").

Why is the variable $grav undefined?

<?php namespace Grav\Common;

use \Grav\Common\Grav;
use \Grav\Common\Page\Page;
use \RocketTheme\Toolbox\ResourceLocator\UniformResourceLocator;
use \Eventviva\ImageResize;

include_once getcwd().'/user/plugins/resizer/lib/ImageResize.php';

class TwigResizerFilters extends \Twig_Extension
{
    private $grav;

    public function __construct() {
            $this->grav = Grav::instance();
    }

    public function getName() {
        return 'TwigResizeFilters';
    }

    public function getFilters() {
        return [
            new \Twig_SimpleFilter( 'resize', [$this, 'resizeImage'] )
        ];
    }

    public function resizeImage($mediapath, $maxWidth = 1920) {

        if (file_exists($mediapath)) {
            // if file exists

            if ($currImg = getimagesize($mediapath)) {
                // if is image

                if (preg_match('(jpg|jpeg|png)', $currImg['mime'])) {
                    // if file format correct

                    $resizedFolder = 'images/resized/';

                    // calculate exact img dimensions for proper naming
                    $maxHeight = floor(($maxWidth/$currImg[0]) * $currImg[1]);


                    if (!file_exists($resizedFolder)) {
                        // create folder if it does not exist
                        mkdir($resizedFolder, 0777, true);
                    }

                    // create filename
                    $resizedExtension = '.'.pathinfo($mediapath, PATHINFO_EXTENSION);
                    $resizedFilename = basename($mediapath, $resizedExtension).'@'.$maxWidth.'x'.$maxHeight.$resizedExtension;

                    if (file_exists($resizedFolder.$resizedFilename)) {
                        // if file already has been cached, just potput
                        return $resizedFolder.$resizedFilename;
                    } else {
                        // if not cached, resize to desired size

                        $image = new ImageResize($mediapath);
                        $image->resize($maxWidth, $maxHeight);
                        $image->save($resizedFolder.$resizedFilename);

                        return $resizedFolder.$resizedFilename;
                    }

                } else {
                    $grav['debugger']->addMessage("File type of ".$mediapath." is not supported.");
                }

            } else {
                $grav['debugger']->addMessage($mediapath." is not an image.");
            }

        } else {
            $grav['debugger']->addMessage("File ".$mediapath." does not exist.");
        }
    }

    private function mergeConfig( Page $page ) {
        $defaults = (array) $this->grav['config']->get('plugins.resizer');
        if ( isset($page->header()->resizer) ) {
            $this->grav['config']->set('plugins.resizer', array_merge($defaults, $page->header()->resizer));
        }
    }
}