I added some extra repositories with the Software Sources program. But when I reload the package database, I get an error like the following: W: GPG error: http://ppa.launchpad.net trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8BAF9A6F I know I can fix it using apt-key in a terminal, according to the official Ubuntu documentation

