Impossible d'uploader un fichier avec Symfony 2

Bonjour à tous,

J'ai besoin d'une aide parce que je sèche complètement sur un problème d'upload de fichier pour un document avec Symfony 2

Je vais déjà vous mettre le code :

Entité : Document.php

<?php

namespace AppBundle\Entity;

use DateTime;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
use Symfony\Component\HttpFoundation\File\UploadedFile;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * Info
 *
 * @ORM\Table(name="document")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\DocumentRepository")
 * @ORM\HasLifecycleCallbacks()
 * @UniqueEntity(fields="title", message="Un document existe déjà avec ce titre.")
 */
class Document {

    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="title", type="string", length=255)
     * @Assert\NotBlank()
     */
    private $title;

    /**
     * @var string
     *
     * @ORM\Column(name="resume", type="text", nullable=true)
     */
    private $resume;

    /**
     * @var DateTime
     *
     * @ORM\Column(name="date", type="date", nullable=true)
     */
    private $date;

    /**
     * @var String 
     * @ORM\Column(name="path", type="text", length=255, nullable=true)
     */
    private $path;

    /**
     * @Assert\File(maxSize="500k")
     */
    private $file;

    /**
     *
     * @ORM\ManyToOne(targetEntity="DocumentType", inversedBy="documents", cascade={"persist"})
     * @ORM\JoinColumn(nullable=true, onDelete="SET NULL")
     *
     * */
    private $documentType;

    /**
     * @ORM\ManyToMany(targetEntity="Author", inversedBy="documents", cascade={"persist"})
     * @ORM\JoinColumn(nullable=true, onDelete="SET NULL")
     * */
    private $authors;

    /**
     * @ORM\ManyToMany(targetEntity="Event", inversedBy="documents", cascade={"persist"})
     * @ORM\JoinColumn(nullable=true, onDelete="SET NULL")
     * */
    private $events;

    /**
     * @ORM\ManyToMany(targetEntity="Tag", inversedBy="documents", cascade={"persist"})
     * @ORM\JoinColumn(nullable=true, onDelete="SET NULL")
     */
    private $tags;

    /**
     * @var DateTime2
     *
     * @ORM\Column(name="creation_date", type="datetime", nullable=true)
     * @Assert\DateTime()
     */
    private $creationDate;

    /**
     * @var DateTime2
     *
     * @ORM\Column(name="last_updated", type="datetime", nullable=true)
     * @Assert\DateTime()
     */
    private $lastUpdated;

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

    /**
     * Constructor
     */
    public function __construct() {
        $this->tags = new ArrayCollection();
        $this->authors = new ArrayCollection();
        $this->events = new ArrayCollection();
    }

    /**
     * Set title
     *
     * @param string $title
     *
     * @return Document
     */
    public function setTitle($title) {
        $this->title = $title;

        return $this;
    }

    /**
     * Get title
     *
     * @return string
     */
    public function getTitle() {
        return $this->title;
    }

    /**
     * Set resume
     *
     * @param string $resume
     *
     * @return Document
     */
    public function setResume($resume) {
        $this->resume = $resume;

        return $this;
    }

    /**
     * Get resume
     *
     * @return string
     */
    public function getResume() {
        return $this->resume;
    }

    /**
     * Set date
     *
     * @param DateTime $date
     *
     * @return Document
     */
    public function setDate($date) {
        $this->date = $date;

        return $this;
    }

    /**
     * Get date
     *
     * @return DateTime
     */
    public function getDate() {
        return $this->date;
    }

    /**
     * Add tag
     *
     * @param Tag $tag
     *
     * @return Document
     */
    public function addTag(Tag $tag) {
        if (!$this->tags->contains($tag)) {
            $tag->getDocuments()->add($this);
            $this->tags->add($tag);
        }

        return $this;
    }

    /**
     * Remove tag
     *
     * @param Tag $tag
     */
    public function removeTag(Tag $tag) {
        if ($this->tags->contains($tag)) {
            $tag->getDocuments()->removeElement($this);
            $this->tags->removeElement($tag);
        }

        return $this;
    }

    /**
     * Get tags
     *
     * @return Collection
     */
    public function getTags() {
        return $this->tags;
    }

    /**
     * Set documentType
     *
     * @param DocumentType $documentType
     *
     * @return Document
     */
    public function setDocumentType(DocumentType $documentType = null) {
        $this->documentType = $documentType;

        return $this;
    }

    /**
     * Get documentType
     *
     * @return DocumentType
     */
    public function getDocumentType() {
        return $this->documentType;
    }

    /**
     * Add author
     *
     * @param Author $author
     *
     * @return Document
     */
    public function addAuthor(Author $author) {
        if (!$this->authors->contains($author)) {
            $author->getDocuments()->add($this);
            $this->authors->add($author);
        }

        return $this;
    }

    /**
     * Remove author
     *
     * @param Author $author
     */
    public function removeAuthor(Author $author) {
        if ($this->authors->contains($author)) {
            $author->getDocuments()->removeElement($this);
            $this->authors->removeElement($author);
        }

        return $this;
    }

    /**
     * Get authors
     *
     * @return Collection
     */
    public function getAuthors() {
        return $this->authors;
    }

    /**
     * Add event
     *
     * @param Event $event
     *
     * @return Document
     */
    public function addEvent(Event $event) {
        if (!$this->events->contains($event)) {
            $event->getDocuments()->add($this);
            $this->events->add($event);
        }

        return $this;
    }

    /**
     * Remove event
     *
     * @param Event $event
     */
    public function removeEvent(Event $event) {
        if ($this->events->contains($event)) {
            $event->getDocuments()->removeElement($this);
            $this->events->removeElement($event);
        }

        return $this;
    }

    /**
     * Get events
     *
     * @return Collection
     */
    public function getEvents() {
        return $this->events;
    }

    /**
     * Sets file.
     *
     * @param UploadedFile $file
     */
    public function setFile(UploadedFile $file = null) {
        $this->file = $file;
    }

    /**
     * Get file.
     *
     * @return UploadedFile
     */
    public function getFile() {
        return $this->file;
    }

    public function getAbsolutePath() {
        return null === $this->path ? null : $this->getUploadRootDir() . '/' . $this->path;
    }

    public function getWebPath() {
        return null === $this->path ? null : $this->getUploadDir() . '/' . $this->path;
    }

    protected function getUploadRootDir() {
        // the absolute directory path where uploaded
        // documents should be saved
        return __DIR__ . '/../../../../web/' . $this->getUploadDir();
    }

    protected function getUploadDir() {
        // get rid of the __DIR__ so it doesn't screw up
        // when displaying uploaded doc/image in the view.
        return 'img/uploads/avatars';
    }

    public function upload() {
        // the file property can be empty if the field is not required
        //var_dump($this->getFile());die();
        if (null === $this->getFile()) {
            return;
        }

        // use the original file name here but you shoul    d
        // sanitize it at least to avoid any security issues
        // move takes the target directory and then the
        // target filename to move to
        $this->getFile()->move(
                $this->getUploadRootDir(), $this->getFile()->getClientOriginalName()
        );

        // set the path property to the filename where you've saved the file
        $this->path = $this->getFile()->getClientOriginalName();

        // clean up the file property as you won't need it anymore
        $this->file = null;
    }

    /**
     * Set creationDate
     * @ORM\PrePersist()
     * @return Document
     */
    public function setCreationDate() {
        $this->creationDate = new \DateTime();

        return $this;
    }

    /**
     * Get creationDate
     *
     * @return DateTime
     */
    public function getCreationDate() {
        return $this->creationDate;
    }

    /**
     * Set lastUpdated
     *
     * @ORM\PrePersist()
     * @ORM\PreFlush()
     * 
     * @return Document
     */
    public function setLastUpdated() {
        $this->lastUpdated = new \DateTime();

        return $this;
    }

    /**
     * Get lastUpdated
     *
     * @return DateTime
     */
    public function getLastUpdated() {
        return $this->lastUpdated;
    }

    /**
     * Set path
     *
     * @param string $path
     *
     * @return Document
     */
    public function setPath($path)
    {
        $this->path = $path;

        return $this;
    }

    /**
     * Get path
     *
     * @return string
     */
    public function getPath()
    {
        return $this->path;
    }
}

Controller : DocumentController.php

/**
     * @Route("/editer/document/{id}", name="editDocument")
     */
    public function editDocumentAction(Request $request, $id) {

        $logger = $this->get('logger');

        $em = $this->getDoctrine()->getManager();
        $document = $em->getRepository('AppBundle:Document')->findOneById($id);
        $form = $this->createForm(new DocumentType(), $document);
        $form->handleRequest($request);

        if ($form->isValid()) {

            $em = $this->getDoctrine()->getManager();

            $document->uploadFileDocument();
            $em->persist($document);
            $em->flush();

            $info = $this->get('translator')->trans('document.alerts.modified', array('%entityName%' => $document->getTitle()));
            $request->getSession()->getFlashBag()->add('info', $info);
            return new JsonResponse(array('message' => 'Success!'), 200);
        } else {

            $response = new JsonResponse(array(
                'form' => $this->renderView('Document/Forms/form-add-document.html.twig', array('form' => $form->createView(),
                    'action' => $this->generateUrl('editDocument', array('id' => $id))))), 400);
            return $response;
        }
    }

DocumentType.php :

<?php

namespace AppBundle\Form;

use Symfony\Bridge\Doctrine\Form\Type\EntityType;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\DateType;
use Symfony\Component\Form\Extension\Core\Type\FileType;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;
use Symfony\Component\Form\Extension\Core\Type\TextareaType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class DocumentType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('title', TextType::class,
                    array('label' => 'document.form.label.title'))
            ->add('resume', TextareaType::class,
                    array('label' => 'document.form.label.resume'))
            ->add('date', DateType::class, array(
                'label' => 'document.form.label.date',
                'widget'      => 'single_text',
                'format'      => 'dd/MM/yyyy'
            ))
            ->add('events', EntityType::class, array(
                'label' => 'document.form.label.events',
                'class'    => 'AppBundle:Event',
                'choice_label' => 'name',
                'multiple' => true,
                'required' => false))
            ->add('authors', EntityType::class, array(
                'label' => 'document.form.label.authors',
                'class'    => 'AppBundle:Author',
                'choice_label' => function ($allChoices) {
                    $pseudos = '';
                    if(!empty($allChoices->getPseudos())) {
                        $pseudos = '(' . $allChoices->getPseudos() . ')';
                    }
                    return $allChoices->getName() . ' ' . $pseudos;
                },
                'multiple' => true))
            ->add('documentType', EntityType::class, array(
                'label' => 'document.form.label.type',
                'class'    => 'AppBundle:DocumentType',
                'choice_label' => 'name',
                'multiple' => false,
                'required'=>false))   
            ->add('tags', EntityType::class, array(
                'label' => 'document.form.label.tags',
                'class'    => 'AppBundle:Tag',
                'choice_label' => 'name',
                'multiple' => true,
                'required'=>false))
            ->add("file", 'file', array(
                'label' => 'document.form.label.file',
                'required' => false))
            ->add('save', SubmitType::class, array('label' => 'Valider'))
        ;
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\Document'
        ));
    }
}

L'erreur que j'ai est :

Error: Call to a member function move() on null" at C:\wamp64\www\interface\interface\src\AppBundle\Entity\Document.php line 496 {"exception":"[object] (Symfony\Component\Debug\Exception\FatalErrorException(code: 0): Error: Call to a member function move() on null at C:\wamp64\www\interface\interface\src\AppBundle\Entity\Document.php:496)"} []

Je ne comprends pas pourquoi mon champ "file" est null, il est bien de type 'file'.
Merci d'avance pour votre aide !