Up to now we have been doing GET operations. Now we are going to start changing things with PUT operations and you will make mistakes as you learn. You might want to create another Mist Org for your testing. In this case I was changing Guest Portal and I made mistake.
So when I went to the UI and clicked on this WLAN I saw:
Not to worry. You just need to open a new window and delete the WLAN without clicking on it — and try again!!!