VirtualBox Machine multiple "empty" icons on Panel

[quasidynamic]

Ubuntu 18.1 with Cinnamon Desktop. The "VirtualBox Machine" icons which are added as tasks to the Task Panel propagate and show several over time. Not sure what is happening to cause the task icons to be duplicated. But even when I close the VM completely, they persist. Click on them does nothing (because they aren't actually running). Eventually I have to restart to clear them. See screenshots.

Any idea how to stop this??

[quasidynamic]

Update: i have narrowed this down to Icing Task Manager panel app for cinnamon desktop. I can clear these icons by toggling "include all app windows" and "only list windows from the monitor displaying this instance" but the results are inconsistent, so i suspect a bug in Icing Task Manager.