I ran into the same issue when tried to debug a 2nd widget in our app. To fix the issue go to Edit Scheme, select Run on the left panel, and then at the top select the Arguments tab. Assign a string value of the widget you want to test as the value of _XCWidgetKind and check the box on the left hand side. Now you can build and test your widget.

So I rewrote the ObjC ViewController being referenced into Swift and am still seeing the same crash: ** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Could not find a storyboard named '(class name)' in bundle NSBundle </private/var/containers/Bundle/Application/0499AB80-469B-4E36-83E2-32D51941809A/(project name).app> (loaded)' So I guess Swift packages can't use storyboard references to other storyboards within the package?