It is not possible to do a reverse match on the default SiteAccess if it doesn't have any matching configuration.

In such case, when previewing content the following exception is thrown:

Type error: Argument 1 passed to eZ\Publish\Core\MVC\Symfony\Event\ScopeChangeEvent::__construct() must be an instance of eZ\Publish\Core\MVC\Symfony\SiteAccess, null given, called in /var/www/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/Helper/ContentPreviewHelper.php on line 82

Reason is that when reverse matching fails, null is returned by the SiteAccess router, while the default one should be returned instead.