Software & Updates hangs and won't close if IPv6 but no IPv4 connectivity

[John Gilmore]

I was running Ubuntu 20.04 beta x86_64 desktop without installing it. Went into Settings, to About, clicked "Software Updates", and up popped this window. My LAN has public IPv6 connectivity but this machine's ethernet address was configured with an IPv4 address that doesn't have global connectivity, just local ethernet. The window hung and would not close when I clicked the [X] in the upper right corner. (Eventually I got the "Force Quit?" pop-up from the window manager and was able to successfully force it to quit.) But -- its UI should always be responsive, no matter what is happening on the network.

The bug-filing page here:

  https://bugs.launchpad.net/ubuntu/+source/software-properties/+filebug/1a28ffd4-807c-11ea-a586-0025b3df357a?

told me:
We also need:

1) The release of Ubuntu you are using, via 'lsb_release -rd' or System -> About Ubuntu
2) The version of the package you are using, via 'apt-cache policy pkgname' or by checking in Software Center
3) What you expected to happen
4) What happened instead

But lsb-release isn't on this (uninstalled live iso) system:

ubuntu@ubuntu:~$ lsb-release -rd

Command 'lsb-release' not found, did you mean:

  command 'lsb_release' from deb lsb-release (11.1.0ubuntu2)

Try: sudo apt install <deb name>

ubuntu@ubuntu:~$

I dno't know what "System -> About Ubuntu" means. There isn't a "System" menu visible. I did go to Settings -> About and it says "Ubuntu Focal Fossa (development branch)" -- but it won't let me select and copy that string!

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: software-properties-gtk 0.98.7
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu22
Architecture: amd64
CasperVersion: 1.442
CurrentDesktop: ubuntu:GNOME
Date: Fri Apr 17 07:21:25 2020
ExecutablePath: /usr/bin/software-properties-gtk
InterpreterPath: /usr/bin/python3.8
LiveMediaBuild: Ubuntu 20.04 LTS "Focal Fossa" - Beta amd64 (20200402)
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
Python3Details: /usr/bin/python3.8, Python 3.8.2, python3-minimal, 3.8.2-0ubuntu1
PythonDetails: N/A
SourcePackage: software-properties
UpgradeStatus: No upgrade log present (probably fresh install)