Preview tab in Query block is showing always NULL for all fields

I've created a Connector (source only) that implements the ExecuteQuery and when I try it in the Preview tab in the query block the value for all fields is always NULL. 

I created a log and the IEnumerable<DataEntity> collection has all the properties with a respective value just before the return of the ExecuteQuery method.

See attached files with more info/details.

What am I missing to make the PREVIEW tab to show the real values of each property?


(1) Answer