I get the same NullReferenceException when I try to customize the CellTemplate, and with other controls when I customize appearance.
What I have discovered is that using stable Xamarin.Forms 184.108.40.2069999 there is no issue but with 220.127.116.11955-pre2 I get the Exceptions. Seems there is a breaking change coming to many Syncfusion components.
ASArun siva Syncfusion Team January 17, 2019 08:14 AM
Thanks for contacting Syncfusion support,
We have analyzed the reported issue “NullReferenceException throws when sets header view for calendar with Xamarin forms version 18.104.22.168955 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 (22.214.171.1249999) of Xamarin Forms, in which it works fine without the exception.