Exception thrown when a registry page's data class removes its interface

[robbieaverill]

CWP 2.3.x-dev

If you create a Registry Page and give it a Data Class of an object that implements RegistryDataInterface, save or publish the page, then remove the interface from that class, the page throws an exception in CMS preview and the frontend.

When you save the page in the CMS it then resets the data class to null and renders "No content".

Kind of edge case behaviour, but we should probably check if the data class implements the RegistryDataInterface before trying to use it, and otherwise render the "No content" message.