Why do I see a "There is no application set" error in BusyContacts?

Certain features, such as creating a To Do from BusyContacts, requires the presence of BusyCal

If you do not have BusyCal installed, you will see a cryptic error from the System which essentially means there's no application available (i.e. BusyCal) to serve your request. You can resolve this by downloading BusyCal.