(captiveportal, new) failsafe, don't try to authenticate if handler not found

src/opnsense/mvc/app/controllers/OPNsense/CaptivePortal/Api/AccessController.php

@@ -141,6 +141,7 @@ class AccessController extends ApiControllerBase
                     $authFactory = new AuthenticationFactory();
                     foreach (explode(',', (string)$cpZone->authservers) as $authServerName) {
                         $authServer = $authFactory->get(trim($authServerName));
+                        if ($authServer != null) {
                             // try this auth method
                             $isAuthenticated = $authServer->authenticate(
                                 $userName,
@@ -152,6 +153,7 @@ class AccessController extends ApiControllerBase
                                 break;
                             }
                         }
+                    }
                 } else {
                     // no authentication needed, set username to "anonymous@ip"
                     $userName = "anonymous@" . $clientIp;