[4.0] Codemirror plugin settings (preview)

[C-Lodder]

Steps to reproduce the issue

1. Go to your Plugin Manager and open the Codemirror plugin
2. Navigate to the "Appearance Options" tab
3. Select any theme from the dropdown

Expected result

Live preview

Actual result

Required to click "Save" to view the preview.

[brianteeman]

Probably related to this js error