Adaptive form data model not showing for sample form



I have installed AEM 6.0 quickstart, and trying the example described here:


I am able to open the adaptive form for edit, however the adaptive form data model tab (on left hand side) is blank.  Also, there is a warning: "Unable to generate JSON from Template provided in Adaptive Form.  Please see the server logs for more Information".

I couldn't find the source of the error in the logs.

Has anyone had issues using the sample adaptive form described in the link above?