Hi @dezbat, I just tried this and the error seems to be avoided if you use the Quark as the named theme to extend in your mytheme.php file:
<?php
namespace Grav\Theme;
use Grav\Common\Theme;
class myTheme extends Quark
{
}
?>
More info at: https://learn.getgrav.org/themes/customization#theme-inheritance
Does doing the same solve your issue?