public function group($name, Closure $callback = null)
    {
        if ($this->groups->has($name)) {
            $group = $this->groups->get($name);
        } else {
            $group = $this->container->make('Maatwebsite\Sidebar\Group');
            $group->name($name);
        }

        $this->call($callback, $group);

        $this->addGroup($group);

        return $group;
    }