NullPointerException during selection of binded ComboBox

Vaadin 6 / 7 / 8 Framework Data Binding
1

Hi, I have a ComboBox binded to bean with property myType of type MyType, the items are supplied by a collection of MyType. The items are loaded correctly, the selected item are correct and match the value of property, but, when I select a new item from dropdown list, the bean is upgrated correctly with new value but the application raise an exception:

java.lang.NullPointerException
    at com.vaadin.data.Converter$2.convertToModel(Converter.java:167)

I have try to defining a Converter, but the exception is raised after the call of convertToModel method.
I ported existing and working code from V7 to V8.
I still don’t understand where I wrong or what I missing.

Cristian