DevTools no longer opens in browser if a previous window was closed

- run a debug session
- run the **Dart: Open DevTools** command
- select  **Open DevTools in Web Browser**
- close the browser window
- try to open again in the browser
- (nothing happens)

I think the DevTools server is not correctly handling the browser being closed, so it tries to reuse a connected client that doesn't exist (rather than launch a new one).