Bugfix: Reference does not implement ArrayAccess

src/Surfnet/StepupMiddleware/CommandHandlingBundle/DependencyInjection/CompilerPass/AddPipelineStagesCompilerPass.php

@@ -44,7 +44,7 @@ public function process(ContainerBuilder $container)
                     . '"%s" is already registered at that position',
                     $stageServiceId,
                     $tagAttributes['priority'],
-                    (string) $prioritized[$priority][0]
+                    (string) $prioritized[$priority]
                 ));
             }