The app certainly should not crash if you have no calendars available, we’ll fix that and instead warn you. Having said that, in order for Agenda to see your calendars and events you’ll need to set them up in the System preferences under accounts. For example, Fantastical talks directly to iCloud and Google, but Agenda needs to do this through the system. Once you have set the accounts up in the system preferences (you can verify that the calendars should now also show in the system Calendar app), you should see them in Agenda as well. If you then make or change an event, the system will pus them to iCloud and Fantastical in turn will sync as well with iCloud and show the changes.