Ubuntu PPA is broken

In the readme there are instructions for adding the PPA:

You can add the this PPA. It gets automatically rebuild every day.

sudo add-apt-repository ppa:shortwave-developers/shortwave-nightly
sudo apt update
sudo apt install shortwave

I get the following error:

E: The repository 'http://ppa.launchpad.net/shortwave-developers/shortwave-nightly/ubuntu bionic Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default.