Email handler: not able to set user defined email body anymore

With the current dev version, I can't select a user specified mail body anymore. The YAML form email handler form (i.e. /admin/structure/yamlform/manage/yaml-form-name/handlers/email-handler-name) can not be submitted. As soon as you use another value from the dropdown, the form can be submitted.

The problem occurred if I create a new email handler or if I want to change an existing email handler which currently uses the default mail body.
Email handlers which already use a user defined mail body (saved with an earlier version of YAML form) can be saved without any problem.