Update

src/Http/ActorReference.php

@@ -18,17 +18,11 @@ class ActorReference
      */
     private $actor;
 
-    /**
-     * @param User $actor
-     */
     public function setActor(User $actor)
     {
         $this->actor = $actor;
     }
 
-    /**
-     * @return User
-     */
     public function getActor(): User
     {
         return $this->actor;

src/Http/Middleware/AuthenticateWithHeader.php

@@ -50,8 +50,6 @@ public function process(Request $request, Handler $handler): Response
                 $request = RequestUtil::withActor($request, $actor);
                 $request = $request->withAttribute('bypassCsrfToken', true);
                 $request = $request->withoutAttribute('session');
-
-                $request->getAttribute('actorReference')->setActor($actor);
             }
         }
 

src/Http/Middleware/AuthenticateWithSession.php

@@ -30,8 +30,6 @@ public function process(Request $request, Handler $handler): Response
 
         $request = RequestUtil::withActor($request, $actor);
 
-        $request->getAttribute('actorReference')->setActor($actor);
-
         return $handler->handle($request);
     }
 

src/Http/Middleware/InjectActorReference.php

@@ -10,16 +10,18 @@
 namespace Flarum\Http\Middleware;
 
 use Flarum\Http\ActorReference;
+use Flarum\Http\RequestUtil;
 use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
 use Psr\Http\Server\MiddlewareInterface as Middleware;
 use Psr\Http\Server\RequestHandlerInterface as Handler;
+use Flarum\User\Guest;
 
 class InjectActorReference implements Middleware
 {
     public function process(Request $request, Handler $handler): Response
     {
-        $request = $request->withAttribute('actorReference', new ActorReference);
+        $request = RequestUtil::withActor($request, new Guest);
 
         return $handler->handle($request);
     }

src/Http/RequestUtil.php

@@ -16,11 +16,23 @@ class RequestUtil
 {
     public static function getActor(Request $request): User
     {
-        return $request->getAttribute('actor');
+        return $request->getAttribute('actorReference')->getActor();
     }
 
     public static function withActor(Request $request, User $actor): Request
     {
-        return $request->withAttribute('actor', $actor);
+        $actorReference = $request->getAttribute('actorReference');
+
+        if (! $actorReference) {
+            $actorReference = new ActorReference;
+            $request = $request->withAttribute('actorReference', $actorReference);
+        }
+
+        $actorReference->setActor($actor);
+
+        // BC Layer
+        $request = $request->withAttribute('actor', $actor);
+
+        return $request;
     }
 }