I have attached a photo taken of my computer screen when I restart the OS, usually after a kernel or video driver update/upgrade. This picture was taken after I opened MC25, waited about 2 minutes, then closed MC25 and restarted the computer. It doesn't matter whether "Caffeine" is running or not - in case anyone thinks that has something to do with it.
Sometimes, instead of just one MC25 icon (as shown in the photo), I see a vertical row of several identical icons and notices. This usually happens after I have been running several days and opening and closing MC25 during that period.
Since I took the photo with my ancient Note3 phone it not a very good picture, but it shows that MC25 is doing something to prevent a restart until I hit the OK to continue button. Before restarting the computer I did a check of the running services and MC was not running or listed in the System Monitor, so I don't know why it's suddenly appearing during attempts to restart.
This has been happening since about MC 25.0.83 or 85, not sure which one.
I'm running the 5.0.0.25-generic kernel, but it was also happening with the 4.x.x.55 kernel branch before the update from 18.04.2 to 18.04.3 LTS.