diff --git a/src/Symfony/Bundle/FrameworkBundle/FrameworkBundle.php b/src/Symfony/Bundle/FrameworkBundle/FrameworkBundle.php
index 36e5cd4028010..339b28e83c695 100644
--- a/src/Symfony/Bundle/FrameworkBundle/FrameworkBundle.php
+++ b/src/Symfony/Bundle/FrameworkBundle/FrameworkBundle.php
@@ -98,7 +98,13 @@ public function boot()
         $_ENV['DOCTRINE_DEPRECATIONS'] = $_SERVER['DOCTRINE_DEPRECATIONS'] ??= 'trigger';
 
         $handler = ErrorHandler::register(null, false);
-        $this->container->get('debug.error_handler_configurator')->configure($handler);
+
+        // When upgrading an existing Symfony application from 6.2 to 6.3, and
+        // the cache is warmed up, the service is not available yet, so we need
+        // to check if it exists.
+        if ($this->container->has('debug.error_handler_configurator')) {
+            $this->container->get('debug.error_handler_configurator')->configure($handler);
+        }
 
         if ($this->container->getParameter('kernel.http_method_override')) {
             Request::enableHttpMethodParameterOverride();