This overload does not work as it does not set a parameter type on the Action, so activating it with a parameter cannot work. Use the other add_action_with_parameter() overload, which takes a parameter type.
Member Gio::AppInfo::launch_uris (const Glib::ListHandle< std::string >& uris, GAppLaunchContext* launch_context)
Use reset() instead because this leads to confusion with clear() methods on the underlying class. For instance, people use .clear() when they mean ->clear().