Rom pointed in the right direction. The widget uses the Mx 5 Client parameter style to set the origin, i.e. params.store.caller, see:
https://apidocs.mendix.com/5/client/mx.data.html
You can either change the widget and replace the part:
store: {
caller: this.mxform
},
by:
origin: this.mxform,
or ask for a code fix on github.
Strange, I have this same widget working in a popup in 6.9.1. I do have this widget in a user object in the popup screen so that could be the difference.
Regards,
Ronald