protected function check_plugin_is_installed(string $component) : Bool {
        if (!isset($this->checkedplugins[$component])) {
            $pluginmanager = \core_plugin_manager::instance();
            $plugin = $pluginmanager->get_plugin_info($component);
            $this->checkedplugins[$component] = !is_null($plugin);
        }
        return $this->checkedplugins[$component];
    }