Fail to load web page in DevContainer

**Describe the issue**:
When both the Flask development server and the web application are started, open the url http://localhost:3000/, the following error message appears:
**Repro Steps**:

1. Run `azd auth login`.
2. Run `azd up`.
3. Run `cd backend `.
4. Run `python3 -m venv venv` and `source venv/bin/activate`.
5. Run `poetry install`.
6. Run `azd env get-values > .env`.
7. Run `poetry run generate`.
8. Run `cd ../frontend` and `npm install`.
9. Run `cd ../backend` and `python main.py`.
10. Open a new terminal.
11. Run `cd frontend` and `npm run dev`.
12. Open the url http://localhost:3000/.

**Environment**:

- Azd version: azd version 1.9.5 (commit cd2b7af9995d358aab33c782614f801ac1997dde)
- OS: Dev Containers

**Expected behavior**:
The web pag can load normally.

@jongio and @marlenezw for notification.

Reproduce the issue using the listed azd, backend, and frontend commands, then inspect the browser error and the output from backend/main.py and npm run dev. Done means the application loads normally at http://localhost:3000/ in the Dev Container.

Scritto dal modello di indicizzazione a partire dal testo della issue.