"Code Editor out of Sync" triggered by undo of header level switch

[krassowski]

I've just (accidentally) stumbled upon a super-reliable way to reproduce the error on 3.5.x (but it works ok on 4.x):

1. Create a markdown cell with a header:

   # test

2. Render it (press Ctrl + Enter)
3. Change header level with a shortcut (press e.g. 3)
4. Undo the action (press Ctrl + Z)

Traceback:

TypeError: Cannot read properties of null (reading 'contentChanged')
    at d._removeToolbar (362.e321444.js:1:2031)
    at d._onActiveCellChanged (362.e321444.js:1:851)
    at u (index.ts:673:12)
    at n.emit (index.ts:594:9)
    at n.emit (index.ts:208:15)
    at set activeCellIndex [as activeCellIndex] (2030.c768cbf.js:1:61466)
    at e.selectAbove (2030.c768cbf.js:1:8956)
    at execute (4875.b65beb4.js:1:14597)
    at e.execute (index.ts:437:27)
    at e._executeKeyBinding (index.ts:621:10)

Related to #2951. Possibly related to #13686.