Ensure model disposed after saving

[colin-grant-work]

What it does

Fixes a regression from #10562 where settings.json appears in Problems because a transaction fails to dispose of its reference to an editor.

How to test

1. Open the application.
2. Open the user-scoped settings.json file and ensure that it contains problems. (e.g. add a key that is not a preference like "not-a-preference": true
3. Close the file.
4. Open the Problems view.
5. Observe that the file is not shown in the Problems view.
6. Restart the application and open the Problems view.
7. Observe that the file is not shown.

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

[vince-fugnitto]

I confirmed the bug on master and the changes work well 👍