Make sure notebook model created when calling openNotebookDocument

[dhuebner]

What it does

Fixes a bogus behavior when executing openNotebookDocument().
openNotebookDocument should create a NotebookModel (Document) so that a later call of e.g. showNotebookDocument() is able to open a new editor.
A use case is e.g. when you open a notebook file in a text editor and then use Open in Notebook Editor link in this text editor. Before this change text editor will be closed, but no new notebook editor will open.

How to test

In Explorer use "Open with..." to open a notebook file with a text editor use. If notebook extension is running "Open in Notebook Editor" link should become visible in the editor.

Clicking this link should open a new Notebook editor.

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me 👍