Pylance crash when interpreter change on single file mode.

[heejaechang]

Repro steps.

1. Open a py file in single file mode (no open folder)
2. Wait python core ext to discover interpreter (if not set one)
3. Wait pylance to initialize (check python language server output in output window)
4. Change interpreter to other one
5. Wait pylance to restart
6. Repeat 4 and 5 a few times

Expected: just work.
Actual: vscode shows messagebox indicating pylance has thrown an exception

[heejaechang]

I already discussed the issue with kartik