public function refreshToken()
    {
        if (is_null($this->token) || is_null($this->token->getRefreshToken())) {
            return null;
        }

        $grant = new RefreshToken();
        $refreshToken = $this->provider->getAccessToken($grant, [
            'refresh_token' => $this->token->getRefreshToken()
        ]);

        $this->token = $refreshToken;


        return $this->token;
    }