Guake: Can't reset "Toggle Guake visibility" shortcut key

I want to unassign F12 from "Toggle Guake visibility" but pressing backspace doesn't unset it and sets it back to F12. If I set it to another shortcut key (e.g. F2), pressing backspace sets it back to F2.

There doesn't seem to be a way to unset the shortcut key.

* OS: Ubuntu 19.04
* Guake version: this can be reproduced using the version provided by apt as well as when compiling from the source:

Guake Terminal: 3.6.3
VTE: 0.56.1
VTE runtime: 0.56.1
Gtk: 3.24.8

See https://youtu.be/p-94CWgXVM4

Might be related to https://github.com/Guake/guake/issues/1448.