Add Flash messages, set the sessions to the storage during request.

app/src/Bootstrap.php

@@ -55,7 +55,10 @@ class Bootstrap
         $containerBuilder->addDefinitions([
             'config' => $config,
             // more DI stuff
-            
+            'flash' => function () {
+                $messages = [];
+                return new \Slim\Flash\Messages($messages);
+            }
         ]);
 
         return $containerBuilder->build();
@@ -99,6 +102,16 @@ class Bootstrap
              ->addGlobal('site_description', $config->get('site.description'));
         $app->add(TwigMiddleware::create($app, $twig));
 
+        // Flash Messages
+        $app->add(function ($req, $next) {
+            if (session_status() !== PHP_SESSION_ACTIVE) {
+                session_start();
+            }
+
+            $this->get('flash')->__construct($_SESSION);
+
+            return $next->handle($req);
+        });
     }
 
     /**