When using the ez_route() operator without a resource (location, etc) in content preview (legacy admin siteaccess), the request will fail with a PHP fatal error:

PHP Fatal error:  Unsupported operand types in /var/www/ezpublish/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/MVC/Symfony/Routing/Generator/RouteReferenceGenerator.php on line 54

Reference: https://doc.ez.no/display/EZP/RouteReference

Notes:

It seems that there is no '_route_params' attribute (or even '_route' ) under the preview "internal" request, which causes the following php code to be invalid (array + null):

$params += $this->request->attributes->get( '_route_params' );