Undo and Redo stop working after preview mode toggle

The same is described in #2426 and #2467, and both should have been fixed by #2490. It appears that the case below is still valid.

### Expected
One would assume that undo/redo should still work after going to preview mode and back, especially when seeing `Undo` and `Redo` items enabled in the menu.

### Observed
After activating preview, and then going back, undo/redo stops working.

### Reproduced
1. Create a new note
2. Enter some text gradually. Sth like:
```
One
Two
Three
```
3. Use Undo/Redo to make sure it's works
4. Go to preview mode and back
5. Try using Undo/Redo again ~> it doesn't work

### Where did you see the bug
- OS version: Windows 10 Home via Parallels Desktop | macOS 11.2.3 on MBP 16"
- Simplenote app version: 2.12.0-beta1 Desktop

Start by reproducing the sequence in this issue: edit a new note, use Undo/Redo, toggle preview mode, return, and try Undo/Redo again. Review related issues #2426 and #2467 and the attempted fix in #2490. Done means Undo and Redo still work after returning from preview mode, including when their menu items are enabled.

Written by the indexing model from the issue text.