How to restore "Software & Updates" to display content?

Support & Help Requests
#1

I'm runnig Ubuntu Mate 18.04.01 on 8GB of RAM and it is running very smoothly on 8GB,
compared to Ubuntu Mate 16.04 on 4GB.
Semi-accidentally I removed Launchpad PPA key from "Trusted Software" tab in "Software & Updates" window of Administration branch in Menu. So now "Software & Updates" window is displayed blank with no subtabs and is the size of the monitor.
I basically clicked "Remove" several times on this PPA link, than clicked "Restore Defaults" 2 times and 2 times reloaded trusted software list, when the list was updated the "Software & Updates" window was no longer displayed correctly.

I have the screenshot of the removed key and PPA name, along with 80% of other keys and trusted software links that have been displayed in "Trusted Software" tab.
How can the correct "Software & Updates" window be restored?

#2

for a ppa, sudo add-apt-repository ppa:<user>/<ppa-name>
for a key, sudo apt-key add <keyfile>
Can you post the screenshot?

#3

I can. But in the screenshot there is only the correct previous version of the "Authentification" tab with a list of Trusted Software Providers in "Software & Updates" window as accessed under Administration in Menu.

The problem is that the whole "Software & Updates" window with all other tabs present in it is now not displayed, though I didn't make any essential changes in other tabs. After clicking Menu > Administration > Software & Updates a blank "Software & Updates" window is displayed and is sized per dimensions of the whole monitor screen.

I hasn't enabled automatic backups, so I thought I should probably edit some files representing the Trusted Software Providers as listed in "Authentification" tab of the "Software & Updates" window.

#4

This issue is ok now, after updating, as I clicked "Restore defaults" several times in the end.