Notification Content Extension SpringBoard Error

I am trying to get a Notification Content Extension working in my Xamarin iOS app. I had a Notification Service Extension working fine that added an image. However I can not get the content extension to work.

Everything is built fine. All settings match between the app and the extension. All settings are in info.plist. The target platforms match. The app has a reference to the extension and I have not modified the extension from the default mac template.

I checked the iOS log and this is the relevant error:

SpringBoard [com.mydomain.myapp] No extension available for bundle
SpringBoard [com.mydomain.myapp] Error was encountered trying to find service extension: error=Error Domain=UNErrorDomain Code 1904 "Unknown application"

I've tried deleting and readding the extension. I've tried deleting the app and reinstalling.

Almost all the settings except the info.plist settings are the same as the service extension that worked.

Does anyone know what this error means?

Thanks.