There is no main category related to context: default

I want to add a profile picture to my admin class, but i got this error : There is no main category related to context: default I searched a lot but no solution. this is my code config.yml

# app/config/config.yml

sonata_media:
    class:
        media: Application\Sonata\MediaBundle\Entity\Media
        gallery: Application\Sonata\MediaBundle\Entity\Gallery
        gallery_has_media: Application\Sonata\MediaBundle\Entity\GalleryHasMedia
        category:   Application\Sonata\ClassificationBundle\Entity\Category

    default_context: default # you need to set a context
    db_driver: doctrine_orm # or doctrine_mongodb, doctrine_phpcr it is mandatory to choose one here
    contexts:
        default:
            providers:
                - sonata.media.provider.dailymotion
                - sonata.media.provider.youtube
                - sonata.media.provider.image
                - sonata.media.provider.file
                - sonata.media.provider.vimeo

            formats:
                small: { width: 100 , quality: 70}
                big:   { width: 500 , quality: 70}
        # other contexts here
        engine:
            providers:
                - sonata.media.provider.image

            formats:
                preview:   { width: 100, quality: 100}
                small:     { width: 200, quality: 100}
                large:     { width: 600, quality: 100}
    cdn:
        server:
            path: /uploads/media # http://media.sonata-project.org/

    filesystem:
        local:
            directory:  "%kernel.root_dir%/../web/uploads/media"
            create:     false
    providers:
        image:
            service:    sonata.media.provider.image
            resizer:    sonata.media.resizer.square # sonata.media.resizer.square
            filesystem: sonata.media.filesystem.local
            cdn:        sonata.media.cdn.server
            generator:  sonata.media.generator.default
            thumbnail:  sonata.media.thumbnail.format
            allowed_extensions: ['jpg', 'png', 'jpeg']
            allowed_mime_types: ['image/pjpeg', 'image/jpeg', 'image/png', 'image/x-png']
        file:
            service:    sonata.media.provider.file
            resizer:    false
            filesystem: sonata.media.filesystem.local
            cdn:        sonata.media.cdn.server
            generator:  sonata.media.generator.default
            thumbnail:  sonata.media.thumbnail.format
            allowed_extensions: ['pdf', 'txt', 'rtf', 'doc', 'docx', 'xls', 'xlsx', 'ppt', 'pptx', 'odt', 'odg', 'odp', 'ods', 'odc', 'odf', 'odb', 'csv', 'xml', 'sql']
            allowed_mime_types: ['text/csv','application/pdf', 'application/x-pdf', 'application/rtf', 'application/octet-stream', 'text/html', 'text/rtf','application/sql', 'text/plain','application/vnd.openxmlformats-officedocument.spreadsheetml.sheet','application/vnd.ms-excel','application/x-iwork-numbers-sffnumbers','application/vnd.apple.numbers']

doctrine:
    orm:
        entity_managers:
            default:
                mappings:
                    FOSUserBundle: ~
                    SonataMediaBundle: ~
    dbal:
        types: #this is about this line and line below
            json:     \Doctrine\DBAL\Types\StringType

userAdmin

/**
     * @param \Sonata\AdminBundle\Form\FormMapper $formMapper
     *
     * @return void
     */
    protected function configureFormFields(FormMapper $formMapper)
    {
        $formMapper
            ->with('General')
            ->add('username')
            ->add('email')
            ->add('enabled')
            ->add('firstname')
            ->add('lastname')
            ->add('plainPassword', 'password', array(
                'required' => (!$this->getSubject() || is_null($this->getSubject()->getId())),
            ))
            ->add('media', 'sonata_media_type',
                array(
                    'provider' => 'sonata.media.provider.image',
                    'context' => 'default',
                    //'data_class' => 'Application\Sonata\MediaBundle\Entity\Media',
                    'required' => false)
            )
            ->end();
    }

user Entity

class User extends BaseUser
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;


    /**
     * @var
     *
     * @ORM\Column(name="firstname", type="string")
     */
    protected $firstname;


    /**
     * @var
     *
     * @ORM\Column(name="lastname", type="string")
     */
    protected $lastname;

    /**
     * @var \Application\Sonata\MediaBundle\Entity\Media
     * @ORM\ManyToOne(targetEntity="Application\Sonata\MediaBundle\Entity\Media", cascade={"persist"}, fetch="LAZY")
     */
    protected $media;

    /**
     * Set media
     *
     * @param \Application\Sonata\MediaBundle\Entity\Media $media
     * @return User
     */
    public function setMedia(\Application\Sonata\MediaBundle\Entity\Media $media = null)
    {
        $this->media = $media;

        return $this;
    }

    /**
     * Get media
     *
     * @return \Application\Sonata\MediaBundle\Entity\Media
     */
    public function getMedia()
    {
        return $this->media;
    }

    /**
     * Get id
     *
     * @return int
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * @return String
     */
    public function getFirstname()
    {
        return $this->firstname;
    }

    /**
     * @param String $firstname
     */
    public function setFirstname($firstname)
    {
        $this->firstname = $firstname;
    }

    /**
     * @return String
     */
    public function getLastname()
    {
        return $this->lastname;
    }

    /**
     * @param String $lastname
     */
    public function setLastname($lastname)
    {
        $this->lastname = $lastname;
    }
}

any help ? sorry for the english.

On the Github page there is an issue where a contributor suggests running this command:

php app/console sonata:classification:fix-context

The reason you get this error is because you're trying to add media under the context default but the context is not related (linked) to a category.