public function getAccessToken() {
        if (!empty($this->accessToken)) {
            return $this->accessToken;
        }
        $accessToken = $this->sessionStorage->read('access_token');
        if (!empty($accessToken)) {
            $this->accessToken = $accssToken;
            return $this->accessToken;
        }

        $this->exchangeToken();

        return $this->accessToken;
    }