[vscode] Running example FileSystemProvider plugin has different behavior

Bug Description:

When trying to use example FileSystemProvider plugin the (expected) behavior on Code[1] differs from the behavior on Theia[2].

[1] Running plugin command Setup workspace with or without opened workspace creates new and empty directory with name that was passed to vscode.workspace.updateWorkspaceFolders in extension.ts.

[2] Running plugin command Setup workspace without opened workspace throws an error Failed to update workspace folders: Error: There is not active workspace. If any folder was already opened in the workspace, the same plugin command opens new directory, but instead of named, empty one it's system's root.

Steps to Reproduce:

1. Clone microsoft/vscode-extension-samples
2. Add fsprovider-sample plugin to Theia
3. Run Theia
4. Execute Setup workspace command from the command paletter

Additional Information

• Operating System: Ubuntu 18.04 if running on Docker image Arch Linux, kernel 5.7.7 if running on host
• Theia Version: Tried both release 1.3.0 and compiling akosyakov/integrate-vscode-api-7269 branch.