Unattended system upgrade in progress during shutdown; please do not turn off the computer

Possibly relevant information:

Upon enabling Software Updater, the following error message appeared:
E:The repository 'http://ppa.launchpad.net/george-edison55/webp/ubuntu bionic Release' does not have a Release file.
I continued with the Software Updater anyway.

After the requisite reboot, I initiated my customary apt-get update and was greeted with:
Err:2 http://ppa.launchpad.net/george-edison55/webp/ubuntu bionic Release
404 Not Found [IP: 91.189.95.85 80]
E: The repository 'http://ppa.launchpad.net/george-edison55/webp/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.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Upon further investigation, I find that this may have something to do with .webp images and gimp:
https://launchpad.net/~george-edison55/+archive/ubuntu/webp
as I recently added a gimp app that enables processing of images with the .webp extension.

George Langford

installman put his finger on the problem:
http://ppa.launchpad.net/george-edison55/webp/ubuntu/
is the actual webpage URL, bionic release is superfluous
and confuses the Software Updater.

Delving further from the linked page:
http://ppa.launchpad.net/george-edison55/webp/
actually points to:
https://launchpad.net/~george-edison55
and the webp connection appears in a link labelled Webp tools:
https://launchpad.net/~george-edison55/+archive/ubuntu/webp
where the gimp connection is the entire subject:
https://launchpad.net/~george-edison55/+archive/ubuntu/webp

Conclusion: I need to edit the repository in the Software Updater.