As Call me V commented, the solution (devised by actionparsnip) to a very similar problem was to run:
sudo apt-get --reinstall install software-properties-common
Source: Launchpad Question 204415
I recommend that, before running that, you run:
sudo apt-get update
(But go ahead and run the
--reinstall install command even if you get errors in the
There were other solutions discussed earlier in that Launchpad question, so if the above way doesn't work, I recommend checking that, especially if you have (or have ever had) a proxy server configured for web access.
Ultimately, this is a crash--
add-apt-repository should not behave like this, even if something goes wrong when it tries to add the repository. Therefore, if you can still produce this problem, I recommend reporting it as a bug. You can configure Apport to send the important technical information to Launchpad; see How do I submit a full bug report with crash dump and steps to reproduce?