Hi Emil,
You mention both a crosstab and a dropdown box. I assume your SDK component is the crosstab. Is that right?
Normally, the design-time JSON for a data-bound property should contain the selection string for the dataset, which would include the dimension/measure names, so I'm not sure why you're getting a null value. Are you able to share your component.js code to provide a better picture of what might be happening?
Regards,
Mustafa.