The error - in a red warning box - occasionally appears in the Makecode editor.
In connection with the bug I just descibed a few minutes ago, I just tried edtiing my main extension from within the project. I was not editing the second extension (an extension on which the main extension is dependent). When I edited the main extension’s .ts file I got the error message. I went back to the editor home page and selected the main extension to edit it independent of the project. When I did this and then went back to the project, the Makecode editor hung up indefinitely.
Project code lost forever, again, because it’s impossible to reload.