[NotHM]

Thanks for the reply abenedik. I did some tracing and found the culprit.

In case I'm reading this again in the future:
* The library I'm making with this can be run in two environment
 - a development env
 - and the final prod env where it is part of a package of assemblies that installs new UI modules
* The prod env can be set up for different access levels, which requires some XAML and class code.
* The dev env to run the library cannot parse that particular XAML code piece, hence the NullReferenceException error.

Anyhoo, thanks for forcing me to do the tracing. And to my future self, don't panic.