Simultaneous editor changes can throw error in dnd code

[microbit-robert]

This was triggered in micro:bit classroom while trying to reproduce a different error.

If the teacher is editing the student's code, the student can trigger a change that reaches the teacher's editor at the same time that the teacher drags a code snippet over the editor. There is only a small window where this can occur. Most of the time, the code successfully desyncs with the "You have edited a copy of this student’s code" message before the student's change arrives.

The error does not appear to have any knock on effects.

Error:

console.js:45 Error: Unexpected DOM structure
    at KU (dnd.ts:80:11)
    at x6 (dnd.ts:90:3)
    at Object.dragleave (dnd.ts:160:11)