ChoiceBox javadoc contains the following:

"The ChoiceBox can be configured either to support null as a valid choice, or to prohibit it. In the case that it is prohibited, there will always be some item that is selected, as long as there is at least one item defined. By default, no item is selected unless otherwise specified. In the case that null is acceptable, a default entry may be inserted into the list of choices at the top, with a name similar to "None" and localized for different Locales."

However there is nothing in the API that allows specify whether support of prohibit null values.