qtmir: Crash in Lomiri shell when unplugging an external display

How to reproduce this issue:

• Install Debian 13 minimal on physical hardware (e.g. a notebook), and run apt install lomiri afterwards. Then replace lomiri-greeter by package arctica-greeter: apt install arctica-greeter. Reboot.
• Login into Lomiri, attach a second display via HDMI or via a USB-C docking station
• Lomiri should appear on the notebook display and on the external HDMI/USB-C display
• Do some work to assure that Lomiri works, esp. open some apps
• detach the HDMI / USB-C cable of the external display

What you will observe:

• the Lomiri session will crash and be restarted, all previously open apps are gone

What should happen:

• the Lomiri session does not crash when unplugging the external display, all apps are moved over to the remaining display