Hi WebGIS Gdibx,
First, wow - nice app! The cartography looks really good! Any chance I could get your permission to add this app to the Web AppBuilder live sites showcase? If so, can you please send me an email: dlaw@esri.com so we can discuss further.
Second, thanks for describing your issue. Yes, I can confirm that we've identified this as a bug:
> BUG-000107579 The Select widget in Web AppBuilder for ArcGIS displays a blank pop-up for a selected feature of map image layers
We are currently working to address this.
Hope this helps,