Fix fatal exception "Object of class ... could not be converted to string" (#7231)

Co-authored-by: Deeka Wong <huangdijia@gmail.com>

Author: kingIZZZY

src/Server.php

@@ -202,7 +202,7 @@ public function onMessage($server, $frame): void
         $instance = $this->container->get($fdObj->class);
 
         if (! $instance instanceof OnMessageInterface) {
-            $this->logger->warning("{$instance} is not instanceof " . OnMessageInterface::class);
+            $this->logger->warning($instance::class . ' is not instanceof ' . OnMessageInterface::class);
             return;
         }