Fix GPG Error: Missing NO_PUBKEY error on Ubuntu

While I was trying to execute sudo apt-get update on my ubuntu, I saw an error it says something like this:

W: GPG error: [some-website-address] xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY [some-16-characters-long-key]

After some research I fixed problem by finding missed key from http://keyserver.ubuntu.com/, importing it to a file named as key1 and adding it by executing

$ sudo apt-key add key1

command.

Detailed instructions can be found here: http://opensourceforgeeks.blogspot.com/2013/04/w-gpg-error-httpppalaunchpadnet-precise.html