[SuperEditor] Don't restore selection on refocus if selection nodes disappeared from document

[kpsroka]

I have the following use case:

1. I have several nodes in a document,
2. I select some of the nodes,
3. I open a menu that takes focus from SuperEditor, and use a menu item to remove the selected nodes.
4. The menu closes and focus goes back to SuperEditor.

Expected outcome:

• The editor is focused with no selection 👍 .

Actual outcome:

• The editor crashes 💥 .