The "null" values should be handled as follows.

A checkbox allows to set the value to null :
- When the value is null, the checkbox is unchecked.
- Setting a value with the editor checks the checkbox.
- Unchecking the checkbox displays the "null" string.

For ChoiceBox editors, a null value is represented by a "null" item. This item is no visible anymore once an item has been selected.

This impact the following properties:
- Paint inline editor : add a checkbox
- AnchorPane Constraints, GridPane Constraints : the value is set to "null" when the checkbox is unchecked (instead of the previous value dimmed)