Unable to add new items to the custom control field

[daniDevKr]

I have a custom control with a collection-type field.
When i run the app in debug mode and try to add items to the field, the XAML editor show the error:

Error XHR0015 Failed to add HelloItem to ObservableCollection`1. Unable to find element.

Why the error? How to solve the problem?
The source code: https://github.com/jhonToni/CustomComponentApp