Add example about FlashBagAwareSessionInterface

security/access_denied_handler.rst

@@ -46,7 +46,10 @@ unauthenticated user tries to access a protected resource::
         public function start(Request $request, AuthenticationException $authException = null): RedirectResponse
         {
             // add a custom flash message and redirect to the login page
-            $request->getSession()->getFlashBag()->add('note', 'You have to login in order to access this page.');
+            $session = $request->getSession();
+            if ($session instanceof FlashBagAwareSessionInterface) {
+                $session->getFlashBag()->add('note', 'You have to login in order to access this page.');
+            }
 
             return new RedirectResponse($this->urlGenerator->generate('security_login'));
         }

session.rst

@@ -163,6 +163,11 @@ if you type-hint an argument with :class:`Symfony\\Component\\HttpFoundation\\Re
             // the second argument is the value returned when the attribute doesn't exist
             $filters = $session->get('filters', []);
 
+            // when the session is flashBag aware, you can add a flash message
+            if ($session instanceof FlashBagAwareSessionInterface) {
+                $session->getFlashBag()->add('note', 'This is a flash message.');
+            }
+
             // ...
         }
     }