Thanks for letting me know. I’ve re-uploaded the 2.38.1 zip file. If you like you can download it again and re-install it.
I’m assuming that since this is just a warning it’s not impacting the functionality of the plugin but let me know if I’m wrong.
Also: you probably already know this, but you can turn off warnings if you don’t need them on – there’s a setting ‘wp_debug’ in your wp-config.php file and there are error settings in the php.ini file.