Can't remove empty comment thread

[RMacfarlane]

Issue Type: Bug

1. Install extension that uses comments API, ex. github pull requests
2. Click on the gutter to create an empty comment widget
3. Instead of clicking "Add comment", click on the gutter again to collapse the comment widget. Comment widget disappears as expected.
4. Change to a different editor and then back

Empty comment widget reappears.

VS Code version: Code - Insiders 1.37.0-insider (fd59854, 2019-07-18T14:07:42.132Z)
OS version: Darwin x64 18.6.0