PHP Fatal error – get_test_users()

Fatal error while try send testing email for system mails!

[06-Jul-2023 11:56:38 UTC] PHP Fatal error:  Uncaught Error: Call to a member function get_test_users() on string in .../wp-content/plugins/newsletter/subscription/template.php:24
Stack trace:
#0 .../wp-content/plugins/newsletter/includes/module-admin.php(178): require()
#1 .../wp-includes/class-wp-hook.php(308): NewsletterModuleAdmin->menu_page()
#2 .../wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters()
#3 .../wp-includes/plugin.php(517): WP_Hook->do_action()
#4 .../wp-admin/admin.php(259): do_action()
#5 {main}
  thrown in .../wp-content/plugins/newsletter/subscription/template.php on line 24