Fixed a bug when the storage service is initialized later by another bundle.

In this case, we can't add the storage class to the compiled class as the
service does not exists.

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -299,9 +299,14 @@ class FrameworkExtension extends Extension
 
         $this->addClassesToCompile(array(
             'Symfony\\Component\\HttpFoundation\\SessionStorage\\SessionStorageInterface',
-            $container->findDefinition('session.storage')->getClass(),
             $container->getDefinition('session')->getClass(),
         ));
+
+        if ($container->hasDefinition($config['storage_id'])) {
+            $this->addClassesToCompile(array(
+                $container->findDefinition('session.storage')->getClass(),
+            ));
+        }
     }
 
     /**