Cannot paste base64 if filebrowserUploadHandler is set

[chrisooo3]

My simple form looks like this:

final class ContentType extends AbstractType
{
    private RouterInterface $router;

    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        $builder->add('content', CKEditorType::class, [
            'label' => 'Treść',
            'constraints' => [new NotBlank()],
            'config' => [
                'filebrowserUploadHandler' => $this->router->generate('upload')
            ]
        ]);
    }
}

If I want to paste svg image in base64 format, I am getting the following error in dev tool:

Uncaught TypeError: Cannot read property '1' of null

If I remove filebrowserUploadHandler parameter from my ContentType everything works fine, the base64 image is pasted normally.

Does anyone have any idea what happens?