debug: toolbar does not update if focused thread no longer exists

[suzmue]

• VS Code Version: 1.60.1
• OS Version: darwin/amd64

Steps to Reproduce:
Reported in the vscode-go repo: golang/vscode-go#1648

All threads are stopped expected toolbar to be:

Instead got:

This occurs when a stopped event is returned with no threadId AND the previously focused thread no longer exists, then the toolbar is left in the running state with the pause button enabled.

All of the active threads are paused:

But the focused thread is still running, but actually does not exist anymore:

The focused thread should be cleared if the thread no longer exists.

[roblourens]

@suzmue I assume you tested the PR that you sent, but would appreciate you double-checking that it is fixed in our build since I'm not sure how to trigger the issue, thanks!