Don't crash when trying to add calendar from invalid URI
This change fixes a crash that occurs when entering an invalid URI in the "Import a Calendar" section within the "New Calender" dialog.
Steps to reproduce:
- Visit the "New Calendar" dialog, via "Manage Calendars..." -> "Add Calendar..."
- Enter a URI that has an unsupported scheme, and an empty host, for example "caldav:/"
- The app crashes
Expected behavior: The app indicates that the URI is invalid.