无法启动会话：已由 PHP 启动（设置了 _SESSION）。500 内部服务器错误 - 运行时异常

我正在尝试为我的用户管理捆绑实现一个简单的登录页面，我是通过表单构建器创建表单的新手。我的捆绑包中只有3个函数，其中有一个新会话，它在不同的路由中被调用，即使我删除了其他2个，我仍然收到相同的错误。这是我的代码：

    <?php

    namespace Acme\UserManagementBundle\Controller;

    use Symfony\Bundle\FrameworkBundle\Controller\Controller;
    use Symfony\Component\HttpFoundation\Request;
    use Symfony\Component\HttpFoundation\Session\Session;
    use Acme\UserManagementBundle\Entity\Users;
    use Acme\UserManagementBundle\Form\SigninType;

    class DefaultController extends Controller
    {
        public function indexAction()
        {
            $session = new Session();
            $users = new Users();
            $form = $this->createForm(new SigninType(), $users);

            if ($session->has('token'))
            {
                 return $this->render('AcmeUserManagementBundle:Default:home.html.twig', array('token' => $session->get('token')));
            }
            if (!($session->has('alert')))
            {
                $session->set('alert', 'Welcome');
                return $this->render('AcmeUserManagementBundle:Default:index.html.twig', array('form' => $form->createView(), 'alert' => 'Welcome!'));   
            }
            else
            {
                $alert = $session->get('alert');
                $session->clear();
                return $this->render('AcmeUserManagementBundle:Default:index.html.twig', array('form' => $form->createView() ,'alert' => $alert));
            }
        }

        public function logoutAction()
        {
            $session = new Session();
            $session->invalidate();
            return $this->render('AcmeUserManagementBundle:Default:index.html.twig');
        }

         public function signupAction()
        {
            return $this->render('AcmeUserManagementBundle:Default:signup.html.twig');
        }
        public function LoginAction(Request $request)
        {
            $session = new Session();
            if ($request->getmethod()=='POST' || $session->get('token') != ""){
                $user = $request->get('user');
                $password = $request->get('password');
                $em = $this->getDoctrine()->getManager();
                $repository = $em->getRepository('AcmeUserManagementBundle:Users');
                $username = $repository->findOneBy(array('username'=>$user,'password'=>$password));
                    if (!$session->get('token') && $username)
                    {
                       $token = $this->get('token_generator')->getToken();
                       $session->set('token', $token, 'user', $username);
                    } else {
                        $session->set('alert', 'Invalid Username and/or Password!');
                        return $this->redirect($this->generateUrl('homepage'));
                    }
                    return $this->redirect($this->generateUrl('homepage'));
            } else {
                return $this->redirect($this->generateUrl('homepage'));
            }   
        }
    }

当我试图在我的函数中实现一个表单构建器接口时，我突然遇到了这个与从html到html的正常创建，我根本没有遇到任何问题。我的代码有问题吗？因为我通过HTML尝试了这个登录代码，并将其转换为Symfony的formbuilderinterface组件。