Hello @nightsky30, since the crash is in xfce, it should also be brought to the attention of the xfce devs.
Xfce task manager is what needs to be fixed in my opinion. This application does not rely on Steam (runs without Steam) so it should be hardened to not crash from an application that it adds to its lists, charts, or whatever.
In addition, looks like there's a newer version so you should either get that version or wait for that version because it might no longer happen with it https://git.xfce.org/apps/xfce4-taskmanager
I've cloned, built, and installed xfce4-taskmanager v1.2.2 from git head master, and I cannot reproduce the crash with that version.

Closing per bugzilla resolution.
Not to reopen the issue, but I wanted to mention that I recently had to do a clean reinstall of Xubuntu 18.10 with Steam due to hardware failure, and the same behavior occurs again. There has to be some issue with the way that Ubuntu/Xubuntu built xfce4-taskmanager for 18.10 or the dependencies used that jive better with 1.2.2 built from master.
Nothing extracted yet.
Your system information
https://gist.github.com/nightsky30/45ea820b910459d8d6a6e25c7903275a
Please describe your issue in as much detail as possible:
Not sure if this goes here but Steam definitely plays a role...Already had Xubuntu report crashes.
I repeatedly crashed xfce4-taskmanager by starting the XFCE Task Manager, starting the Steam client, and closing the Steam client windows. It does not matter what Steam client window is closed whether it be the "Create/Login existing", "Login credentials", or main main Steam client window. dmesg logs always showed the following when the task manager crashed. Repeated crashes did not change the code produced:
[ 538.849213] xfce4-taskmanag[14268]: segfault at 0 ip 0000558eda955a23 sp 00007ffd6c79c000 error 4 in xfce4-taskmanager[558eda948000+f000]
[ 538.849216] Code: fb 48 83 ec 08 e8 ed 2e ff ff 85 c0 74 09 48 83 c4 08 5b 5d c3 66 90 e8 db 32 ff ff 48 89 df 48 89 c5 e8 b0 36 ff ff 48 89 ee <48> 8b 38 e8 85 29 ff ff 48 83 c4 08 5b 48 89 c7 5d e9 97 3c ff ff
Steps for reproducing this issue: