Your Site is Experiencing a Technical Issue

Hi David,
I upgraded to PHP8 a week or so ago and today got this error / mail.

WordPress version 5.7
Current theme: Independent Publisher (version 1.8)
Current plugin: Post Kinds (version 3.5.1)
PHP version 8.0.3

Error Details
=============
An error of type E_ERROR was caused in line 17 of the file /var/www/domain.tld/wp-content/plugins/indieweb-post-kinds/templates/reply-metabox.php. Error message: Uncaught TypeError: Cannot access offset of type string on string in /var/www/domain.tld/wp-content/plugins/indieweb-post-kinds/templates/reply-metabox.php:17
Stack trace:
#0 /var/www/domain.tld/wp-includes/template.php(730): require_once()
#1 /var/www/domain.tld/wp-content/plugins/indieweb-post-kinds/includes/class-kind-metabox.php(323): load_template()
#2 /var/www/domain.tld/wp-admin/includes/template.php(1389): Kind_Metabox::reply_metabox()
#3 /var/www/domain.tld/wp-content/plugins/indieweb-post-kinds/includes/class-kind-metabox.php(106): do_meta_boxes()
#4 /var/www/domain.tld/wp-includes/class-wp-hook.php(292): Kind_Metabox::after_title_metabox()
#5 /var/www/domain.tld/wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters()
#6 /var/www/domain.tld/wp-includes/plugin.php(484): WP_Hook->do_action()
#7 /var/www/domain.tld/wp-admin/edit-form-advanced.php(581): do_action()
#8 /var/www/domain.tld/wp-admin/post.php(206): require('...')
#9 {main}
  thrown

Thanks for your work with this plugin.
Mark.