public function buildMessage()
    {
        if (isset($this->message)) {
            return $this->message;
        }

        $this->message = [
            BuildUtil::removeNullElements([
                'type' => MessageType::FLEX,
                'altText' => $this->altText,
                'contents' => $this->containerBuilder->build(),
                'quickReply' => BuildUtil::build($this->quickReply, 'buildQuickReply'),
            ])
        ];

        return $this->message;
    }