You can use tags to inject subscriber fields.
Fatal error: Uncaught Error: Call to a member function wp_editor() on null in /home/feesserscom/public_html/wp-content/plugins/newsletter/emails/blocks/text/options.php:17
Stack trace:
#0 {main}
thrown in /home/feesserscom/public_html/wp-content/plugins/newsletter/emails/blocks/text/options.php on line 17