Step to reproduce:

1. Log in, a new message inform about upgrading to 18.04. There is a yellow upgrade icon in the system tray
2. Fill the root file system so that there is just a few hundred of MB:

    $ df -h
    /dev/sda2    17G    9.6G    6.0G    62% /
    $ fallocate -l 5800M temp_fill
    $ df -h
    /dev/sda2    17G     16G    303M    99% /

3. Click on the yellow upgrade icon in the system tray. Click "upgrade" on the upgrade window. The upgrade icon disappears from the system tray. The upgrade will fail because of lack of space.

4. Free some space on the root file system:

    rm temp_fill

5. Notice that there is no more yellow upgrade icon in the system tray. In the notification and status section of the system tray, upgrade section show that the system is up-to-date. Hence there is no easy way to find how to upgrade at that point.

`ps aux | grep distro|` shows that distro-release-notifier is still running in the background. I did not find how to access it though. It is possible to run `sudo distro-release-notifier` but it warns about a missing icon (see below). An "invisible" icon is added to the system tray, when clicking on it, it is possible to start the upgrade again.

    $ distro-release-notifier
    org.kde.distro-release-notifier: Failed to register org.kde.DistroReleaseNotifier
    $ sudo distro-release-notifier
    QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
    QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
    QSystemTrayIcon::setVisitble: No Icon set



Expected result: 
The yellow upgrade icon should automatically return to the system tray after the upgrade fails.

SOFTWARE VERSIONS
(available in About System)
KDE Plasma Version: 5.14.80
KDE Frameworks Version: 5.51.0
Qt Version: 5.11.1
distro-release-notifier Version: 0.0+p16.04+git20180924.1417-0