Expose more Greenworks Steam cloud APIs to NW.Js and Electron (1.4.0: Supports NW.Js 0.82.0, Steam SDK 1.58, Greenworks 0.15, Electron with efc).
Can you try again and make sure you are using 1.4.1? I just tried with 1.4.1 and it looked ok. If there is still an error, open the dev console and send the details of the error.