Audio ist not generated because $wp_filesystem is a boolean

Replies: 0

I was testing this plugin but the audio was not generated. Instead I got an exception in the logfile:

[06-Mar-2019 13:58:06 UTC] PHP Fatal error:  Uncaught Error: Call to a member function exists() on boolean in htdocs/wp-content/plugins/amazon-polly/admin/AmazonAI-PollyService.php:299

Turns out that this is called by generate_audio which calls $common->prepare_wp_filesystem. This is supposed to return an object I assume.

In here I find this line (line 1021 in admin/AmazonAI-Common.php):

        if ( ! WP_Filesystem( $creds ) ) {
            request_filesystem_credentials( $url );
            return true;
        }

Is this correct? Because the true is not what is needed. The other options actually return $wp_filesystem

So how do I solve this problem?