Build status reporter should work well multi-root workspace

[akaroml]

When I "File->Add Folder to Workspace" for the first time to add a Java project, an untitled workspace is created and vscode tries to create a second instance of the server status window. This results in the attached error dialog. Note that "frontend" is the first java project, and "backend" is the second project that was added to the workspace, causing the error.

Originally posted by @jclingan in #1153 (comment)

[akaroml]

@fbricon I'll also try fix this one. Please help assign to me.