Renderer should handle nested editables [FF] ckeditor5#721

TC1

1. Put the caret in the first paragraph and type something.
2. Put the caret inside the widget (in "bar").
3. Click "Undo" or press Ctrl+Z (check both).

Expected:

No error in the console.

TC2

Try the same TC as above but use both nested editables. See if the focus is correctly moved between them.