Ubuntu 20.04: Cannot start gnome-tweak-tool

[moma]

$ sudo apt install gnome-tweak-tool

$ gnome-tweaks
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/gtweak/app.py", line 30, in do_activate
    self.win = Window(self, model)
  File "/usr/lib/python3/dist-packages/gtweak/tweakview.py", line 25, in __init__
    self.main_box = Handy.Leaflet()
  File "/usr/lib/python3/dist-packages/gi/module.py", line 176, in __getattr__
    interfaces = tuple(interface for interface in get_interfaces_for_object(info)
  File "/usr/lib/python3/dist-packages/gi/module.py", line 102, in get_interfaces_for_object
    interfaces.append(getattr(module, name))
  File "/usr/lib/python3/dist-packages/gi/module.py", line 215, in __getattr__
    wrapper = metaclass(name, bases, dict_)
  File "/usr/lib/python3/dist-packages/gi/types.py", line 250, in __init__
    register_interface_info(cls.__info__.get_g_type())
TypeError: must be an interface
-------------------------

The application does not appear.
It crashed.

$ gnome-tweaks --version
3.34.0

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04 LTS
Release: 20.04
Codename: focal

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gnome-tweak-tool (not installed)
ProcVersionSignature: Ubuntu 5.4.0-25.29-generic 5.4.30
Uname: Linux 5.4.0-25-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Thu Apr 23 10:10:07 2020
InstallationDate: Installed on 2019-10-29 (176 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Alpha amd64 (20191028)
SourcePackage: gnome-tweaks
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.