Update "Widget Details Tree" when pub root dirs change

[CoderDake]

when the pub root directories change, if the node doesn't exist anymore we should clear the widget details tree pane

[srawlins]

Closing as resolved: DevTools now listens for changes to preferences.inspector.pubRootDirectories and automatically reloads the Inspector tree via controller.refreshInspector(), resetting any outdated node selections.

If there is more work to do here, please let us know by filing a new issue with up to date information. Thanks!