Behaviour "Setting a default Description" no longer works when Rich Text Editor is disabled

Server side behaviour for setting description field will no longer work when Rich Text Editor is disabled.

Reproduction steps:

1. Go to Administration > System > Rich text editor, and disable it for all users
2. Create  behaviour with server side initialization script to set default value of description field (https://scriptrunner.adaptavist.com/5.6.7/jira/behaviours-overview.html#_setting_a_default_description)
3. Save behaviour
4. Add it to all projects and all issue types
5. Go to create issue
6. Description field is empty, but it should be set  to default value by Behaviours