Empty subject results in error

Problem/Motivation

In forum module comments are used to create discussions. Comment title is NOT mandatory.
If a subject is left empty, Forum Notifications Subscription results in an error:
TypeError: Drupal\forum_notifications_subscription\ForumNotificationsSubscriptionTokenService::replacePlain(): Argument #1 ($plain) must be of type string, null given, called in /var/www/vhosts/cms/docroot/modules/contrib/forum_notifications_subscription/forum_notifications_subscription.module on line 291 in Drupal\forum_notifications_subscription\ForumNotificationsSubscriptionTokenService->replacePlain() (Zeile 18 in /var/www/vhosts/cms/docroot/modules/contrib/forum_notifications_subscription/src/ForumNotificationsSubscriptionTokenService.php).