Don't try to use a refresh_token when refresh_token is blank or missing

+++ b/src/Service/Oauth2ClientService.php
@@ -102,13 +102,19 @@ class Oauth2ClientService extends Oauth2ClientServiceBase {
-        $access_token = $this->grantServices['refresh_token']->getAccessToken($clientId);

I think the issue here is that \Drupal\oauth2_client\Service\Grant\Oauth2ClientGrantServiceInterface::getAccessToken doesn't have a return value and it should.

I'm using the client_credentials grant with my plugin, and it has a similar issue when I do the following, asking for the access token for the first time:

$ drush php
>>>  \Drupal::service('oauth2_client.service')->getAccessToken('dcs_canonical_video_url')->getToken();
PHP Error:  Call to a member function getToken() on null in Psy Shell code on line 1

The \Drupal\oauth2_client\Service\Oauth2ClientService::getAccessToken method is returning null, which I don't think is the intent.

I'll try an alternate patch to illustrate.

How about this?

Ahh sorry about that. Not sure what I did. Perhaps your right and my patch was better suited to the other issue. In any case, I decided to go another route with what I was trying to do and use thephpleague/oauth2-client directly.

Patch #2 almost worked for me.
The first time after the expiration of the token, i got an empty access token.

This is because the getaccesstoken method of the grantservice doesn't return a token. It only sets the token.
So i added one line at the end of your patch to get the real access-token.

    if (empty($access_token)) {
      $this->grantServices['client_credentials']->getAccessToken($clientId);
      $access_token = $this->retrieveAccessToken($clientId);
    }

Oops, i was confused about the used method (getClientCredentialsAccessToken) and the explanation of this module at https://www.drupal.org/docs/8/modules/oauth2-client/oauth2-client-8x-2x.
Especially the last sentence : 'If a token has not been generated, the authentication process will be initiated.'
That doesn't worked when the token was expired. I will reference this on the issue you mentioned and switch to it.