Comment 41 for bug 1782984

tl;dr enormous diff XD

Anyways I guess that kills my theory since both of those are Bionic versions. And you say you have the old libfm which utterly shocks me.

Janos, can you please do like you did with pcmanfm and install the Cosmic version of thunar and let me know if it also fails to crash?
https://launchpad.net/ubuntu/+archive/primary/+files/thunar_1.8.1-1_amd64.deb

Hopefully, it, too, can go without an upgrade to its libs, but if it can't:
https://launchpad.net/ubuntu/+archive/primary/+files/libthunarx-3-0_1.8.1-1_amd64.deb