I learned in another thread that you can edit a custom widget in WAB/server/apps/myAppName/widgets rather than editing the custom widget code in client/stemapp/widgets. This allows me to refresh the instance of WAB to see how my code changed the custom widget. Makes sense.
But it seems like you MUST have a folder with your custom widget name in client/stemapp/widgets for this method to work. If not, I was unable to add my custom widget in WAB's interface. Once I added a copy of my custom widget folder to client/stemapp/widgets I was able to edit the code in /server/apps/myAppName/widgets and see changes on-screen.