Warning: Illegal string offset 'manager'

Warning: Illegal string offset ‘manager’ in /wordpress/wp-content/plugins/subscribe-to-double-opt-in-comments/subscribe-to-doi-comments.php on line 422

The above Warning pops up if I’m looking in the WP Dashboard -Tools – Abo’s
I believe there must be a Bug!!!

https://ww.wp.xz.cn/plugins/subscribe-to-comments/