Sorry for the late reply and the inconvenience.
Do you still see this error?
If yes, could you please enable debug log like below and open the editor again?
The debug log will have some info regarding the error.
Otherwise, right click “open editor” -> inspect element -> you will see a HTML tag with a lenghty value just above.. Double click and empty that value. Save the settings.
Reopen the editor and it should work.