I've followed all the instructions here for creating a custom widget.
I've put my widget in the C:\...\client\stemapp\widgets folder and also added the code below to the config.json file in my C:\...\client\stemapp folder.
When I try to test my widget using this URL http://<my machine>:3344/webapp/?config=config.json I only get a blank screen. Upon checking the errors in the Chrome developer console I see that I have an error saying Failed to load resource: the server responded with a status of 404 (Not Found) followed by this URL http://<my machine>:3344/http://<my machine>:3344/sharing/portals/self?f=json&dojo.preventCache=1426103768809
Also, I am unable to see my custom widget in the Choose Widget panel of the Web AppBuilder.
Can anyone help me with this problem please?