GUI is flickering between two states when using the Fonts tab

[Alistair Turnbull]

1) Core i5 2500; Radeon RX 580 Series (POLARIS10, DRM 3.35.0, 5.4.0-54-generic, LLVM 10.0.0); Ubuntu 20.04.1 LTS; 64-bit; Gnome 3.36.3; Wayland.

2) $ apt-cache policy gnome-tweaks
gnome-tweaks:
  Installed: (none)
  Candidate: 3.30.2-1
  Version table:
     3.30.2-1 500
        500 http://ftp.uk.debian.org/debian stable/main i386 Packages

3) I want to configure my system fonts. I run gnome-tweaks, click on Fonts, and click on the drop-down menu to choose a font.

4) The GUI gets stuck in a loop, flickering between two states. I have made a video: https://www.youtube.com/watch?v=6H4nZlwTTAw

While it's flickering, it spews many copies of a warning to the console:

(gnome-tweaks:34175): Gtk-WARNING **: 20:55:29.349: gtk_widget_size_allocate(): attempt to allocate widget with width -23 and height 19

Needless to say, the tool is not usable like this.