Comment 3 for bug 1807032

I tested this on an Ubuntu Cosmic system on which I installed ubuntu-release-upgrader-gtk version 18.10.11.3 from -proposed. I use a local mirror of the archive and used the right environmental variable when calling check-new-release-gtk to upgrade to disco. I ended up seeing the correct message regarding the mirror check in /var/log/dist-upgrade/main.log and did not receive the dialog about no valid mirror.