symfony输出js和css文件，求大神指导为什么会报错了？

<code>$ composer require symfony/assetic-bundle</code>

<code>// app/AppKernel.php

// ...
class AppKernel extends Kernel
{
    // ...

    public function registerBundles()
    {
        $bundles = array(
            // ...
            new Symfony\Bundle\AsseticBundle\AsseticBundle(),
        );

        // ...
    }
}</code>

<code># app/config/config.yml
assetic:
    debug:          '%kernel.debug%'
    use_controller: '%kernel.debug%'
    filters:
        cssrewrite: ~

# ...
</code>