Hi William,
Thanks for contacting Syncfusion support,
We have analyzed the reported issue “NullReferenceException throws when sets header view for calendar with Xamarin forms version 4.0.0.62955 pre-2” and as this version of Xamarin Forms is a prerelease version, hence it may has some bugs in this case the Platform property of VisualElement return as null, while get the property using Reflection, hence kindly use the latest stable version (3.4.0.1009999) of Xamarin Forms, in which it works fine without the exception.
Kindly revert us if you have any concern.
Regards,
Arun Siva R