[Security]fixed Login when there is no session.

Author: aitboudad

book/security.rst

@@ -449,16 +449,21 @@ Next, create the controller that will display the login form::
                 $error = $request->attributes->get(
                     SecurityContextInterface::AUTHENTICATION_ERROR
                 );
-            } else {
+            } elseif (null !== $session && $session->has(SecurityContextInterface::AUTHENTICATION_ERROR)) {
                 $error = $session->get(SecurityContextInterface::AUTHENTICATION_ERROR);
                 $session->remove(SecurityContextInterface::AUTHENTICATION_ERROR);
+            } else {
+                $error = '';
             }
+            
+            // last username entered by the user
+            $lastUsername = (null === $session) ? '' : $session->get(SecurityContextInterface::LAST_USERNAME);
 
             return $this->render(
                 'AcmeSecurityBundle:Security:login.html.twig',
                 array(
                     // last username entered by the user
-                    'last_username' => $session->get(SecurityContextInterface::LAST_USERNAME),
+                    'last_username' => $lastUsername,
                     'error'         => $error,
                 )
             );