admin.order.edit.search.product.initialize:
    - [onTMP, NORMAL]

    public function onTMP($event)
    {
        $app = $this->app;
        if (●) {
            $app->addError('入力エラーがあります', 'admin');
        }

        return;
    }

$form->getErrors();

admin.order.edit.index.initialize:
    - [onTMP, NORMAL]

    public function onTMP($event)
    {
        $app = $this->app;
        
        $request = $event->getRequest();
        if ('POST' === $request->getMethod()) {
            $form = $event['builder']->getForm();
            $form->handleRequest($request);
            if (!$form->isValid()) {
                $app->addError('入力エラーがあります', 'admin');
            }
        }

        return;
    }