megamenurender.php:36 issue after upgrading to PHP8

[chataros]

json_decode(): Argument #1 ($json) must be of type string, array given
Call stack

| Function | Location

1 | () | JROOT/plugins/system/t3/includes/renderer/megamenurender.php:36
2 | json_decode() | JROOT/plugins/system/t3/includes/renderer/megamenurender.php:36
3 | JDocumentRendererMegamenuRender->render() | JROOT/libraries/src/Document/HtmlDocument.php:561
4 | Joomla\CMS\Document\HtmlDocument->getBuffer() | JROOT/libraries/src/Document/HtmlDocument.php:843
5 | Joomla\CMS\Document\HtmlDocument->_renderTemplate() | JROOT/libraries/src/Document/HtmlDocument.php:629
6 | Joomla\CMS\Document\HtmlDocument->render() | JROOT/libraries/src/Application/CMSApplication.php:1013
7 | Joomla\CMS\Application\CMSApplication->render() | JROOT/libraries/src/Application/SiteApplication.php:710
8 | Joomla\CMS\Application\SiteApplication->render() | JROOT/libraries/src/Application/CMSApplication.php:295
9 | Joomla\CMS\Application\CMSApplication->execute() | JROOT/includes/app.php:61
10 | require_once() | JROOT/index.php:32