We are looking into what is causing this crash. We've been unable to reliably reproduce this bug, if you can get it to crash every time please send an e-mail to firstname.lastname@example.org with the steps to reproduce it. We really appreciate it and want to get this bug fixed for the next update!
For now try adding a new News panel by tapping the plus (+) button at the bottom of the app and swiping to the news panel option. Then try adding your source again and see if that resolves the issue.
If that doesn't work, try force quitting Morning and reopening it. You can force quit and app in iOS by double tapping the home button and once the app switcher comes up, swipe Morning up.