The editor is set to hot-reload-on-save by default so saving while the debug session is starting currently prints an error:

Failed to Hot Reload: app 'null' not found

![image](https://github.com/flutter/flutter/assets/1078012/a125b455-a46d-4993-98d8-5d8ae7237a00)

This change skips the call to `app.restart` if the app hasn't started yet to avoid printing an error.