Well, one way to get rid of it is to turn off the Ace editor, i.e. set the UI to None in the question or its prototype.
But ... you should only be seeing that message very rarely, usually only after a version upgrade. After an upgrade Moodle takes typically 10 - 20 seconds to minimise all files in the Ace editor, so it's unavailable during that time. Once minimised it's cached and should be served in plenty of time. If you refresh the page, after say 30 seconds, is the message still there?
I have seen one case where a very slow server (a tiny microcontroller in a box with no heat sink) was unable to minimise the editor before the request timed out, so that message never went away. What are the specs on your server?