Crashes when detaching an allowed USB device while no GUI session is running

[genodeftest]

Steps to reproduce:

1. Have a computer with usbguard-notifier installed
2. Log in via tty (or SSH) only, not via graphical session. If you were logged in via graphical session, this is your moment to log out.
3. Make sure that usbguard notifier runs by running systemctl --user status usbguard-notifier.service
4. Attach a USB device
5. If necessary, use usbguard list-devices, then usbguard allow-device to allow the device
6. Remove the device

What happens:
Crash. backtrace.txt

What should happen:
No crash

Version info:

usbguard-1.1.3-6.fc43.x86_64
usbguard-notifier-0.1.1-2.fc43.x86_64

This issue has been present for a few years, it is not a recent regression, see this Fedora bug report #2154763.

[genodeftest]

In case you think USBGuard notifier shouldn't even have run in a non-graphical session, #82 might fix this problem.