Critical Error composing newsletter while pressing Next

I’ve now encountered this same fatal error on 2 different WP sites. I set up a new test WP site with nothing else on it and I still get this error:

This is what I get via email from WP:

Error Details
=============
An error of type E_ERROR was caused in line 826 of the file /var/www/newsletter.foo.com/wp-content/plugins/newsletter/includes/composer.php. Error message: Uncaught Error: Typed property DOMText::$wholeText must not be accessed before initialization in /var/www/newsletter.foo.com/wp-content/plugins/newsletter/includes/composer.php:826
Stack trace:
#0 /var/www/newsletter.foo.com/wp-content/plugins/newsletter/includes/composer.php(779): TNP_Composer::process_dom_element()
#1 /var/www/newsletter.foo.com/wp-content/plugins/newsletter/emails/composer.php(103): TNP_Composer::convert_to_text()
#2 /var/www/newsletter.foo.com/wp-content/plugins/newsletter/includes/module-admin.php(177): require(‘…’)
#3 /var/www/newsletter.foo.com/wp-includes/class-wp-hook.php(324): NewsletterModuleAdmin->menu_page()
#4 /var/www/newsletter.foo.com/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()
#5 /var/www/newsletter.foo.com/wp-includes/plugin.php(517): WP_Hook->do_action()
#6 /var/www/newsletter.foo.com/wp-admin/admin.php(259): do_action()
#7 {main}
thrown

This is the inline error message after I press the Next button while composing a newletter:

Fatal error: Uncaught Error: Typed property DOMText::$wholeText must not be accessed before initialization in /var/www/newsletter.foo.com/wp-content/plugins/newsletter/includes/composer.php:826
Stack trace: #0 /var/www/newsletter.foo.com/wp-content/plugins/newsletter/includes/composer.php(779):
TNP_Composer::process_dom_element() #1 /var/www/newsletter.foo.com/wp-content/plugins/newsletter/emails/composer.php(103):
TNP_Composer::convert_to_text() #2 /var/www/newsletter.foo.com/wp-content/plugins/newsletter/includes/module-admin.php(177):
require(‘…’) #3 /var/www/newsletter.foo.com/wp-includes/class-wp-hook.php(324):
NewsletterModuleAdmin->menu_page() #4 /var/www/newsletter.foo.com/wp-includes/class-wp-hook.php(348):
WP_Hook->apply_filters() #5 /var/www/newsletter.foo.com/wp-includes/plugin.php(517):
WP_Hook->do_action() #6 /var/www/newsletter.foo.com/wp-admin/admin.php(259):
do_action() #7 {main} thrown in /var/www/newsletter.foo.com/wp-content/plugins/newsletter/includes/composer.php on line 826