Fix notebook widget disposal

[msujew]

What it does

In case the notebook loading fails, disposing the notebook widget also failed (due to calling disconnect on undefined), which led to a cascade of other issues related to that widget.

How to test

1. Open a notebook file (I created a copy of an existing notebook
2. Close Theia and delete the notebook file
3. Open Theia again, the notebook error marker should be visible on the widget
4. Recreate the original notebook file
5. Close and reopen the widget. It should load the recreated notebook as expected.

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

[jonah-iden]

LGTM 👍