gallery widget not working

When I add a gallery widget like this (image uris are just examples):

var images = new scopes.lib.PreviewWidget("images", "gallery");
images.add_attribute_value("sources",
    [
        'http://.../img1.png',
        'http://.../img2.png'
    ]
);

I get this error on console:
file:///usr/share/unity8//Dash/Previews/PreviewImageGallery.qml:131:25: Unable to assign QJSValue to QUrl
file:///usr/share/unity8//Dash/Previews/PreviewImageGallery.qml:62:21: Unable to assign QJSValue to QUrl

The images are not displayed.

Code example here: http://bazaar.launchpad.net/~m982/+junk/perform-action-bug/view/head:/src/perform-action.js#L167

Using the ubuntu-sdk-15.04.4 framework.