No CSS text area in the Ultimate Branding settings

I go to: mydomain.com/wp-admin/network/admin.php?page=branding&tab=css
But see no text area to put my css code.
I can see errors in the console:

edit_area_full.js:25 Uncaught TypeError: Cannot read property 'length' of undefined
http://mydomain.com/wp-admin/css/colors.css Failed to load resource: the server responded with a status of 404 (Not Found)

(The second error is another error from the plugin I believe. It started after I've set the colors to the Ultimate Branding Color scheme).