Howto: wp_dropdown_categories in widget control

I’m currently creating a news slider widget in which you can show posts from a specific category. I have everything working but the improvement I want in the widget control is that you can select the correct category via a dropdown list instead of having to insert the correct category ID.

I have tried to get wp_dropdown_categories() working in the control section of my widget but nothing appears on the screen… anyone an idea how I can get this working?

Cheers!