Solution of Error Message: "Ignoring file 'XXXXXXXXXXXX.save' in directory '/etc/apt/sources.list.d/' as it has an invalid filename extension"

While using Ubuntu 20.04, I've encountered the warning in the subject line of this post after using update command in terminal. I think it is about some compatibility problem with the sources added after Ubuntu setup. 

I tried to get rid of that message deleting all the ".save" files in /etc/apt/sources.list.d/ directory. But no way... After first use of update command, they have been created again. 

After a little research, I found a solution as a workaround. Writing the below command hides the warning message forever: 

sudo sh -c "echo 'Dir::Ignore-Files-Silently:: \"(.save|.distupgrade)$\";' > /etc/apt/apt.conf.d/99ignoresave"