tumblerd GStreamer plugin crashes on startup, causing desktop session to exit

This concerns tumbler 0.2.8-1 in Ubuntu focal.

Symptom: When I log into the Xubuntu desktop environment, the desktop begins to appear on the screen, but then the X server dies and I am unceremoniously returned to the LightDM login screen.

If I uninstall tumbler, then I can log into the desktop environment without issue.

I performed further testing, with tumbler installed. I looked at the shared libraries in /usr/lib/x86_64-linux-gnu/tumbler-1/plugins/, and experimented with removing/replacing them.

I've confirmed that removing tumbler-gst-thumbnailer.so is enough to make the crash go away. Conversely, leaving it in as the only plugin causes the crash to occur.

Because this library is a D-Bus plugin, I do not know how to debug it. There is no crash file for tumbler in /var/crash/ (but I see ones for xfsettingsd and the Xorg X server).