JavaFX 2.0 :: How To Not Show ListView ContextMenu When Right Clicking On Null

I only want the context menu to be showen if the user is clicking on an actual listview item, not if they click anywhere else in the listview. I know this is possible and I found answers to similar problems, but in the end they didn't provide enough infomation for me to solve my issue. I found this example on how to do it with a list view containing String objects, but what if my list view contains another object, in my case a wrapper class? I have a wrapper class to keep track of the key, but I only display the value variable in the listview, through calling the toString method. So the cells still contain a wrapper.

class Wrapper(int key, String value) {
..
}
public String getValue() { return value; }
public SimpleStringProperty getValueAsProperty() { return new SimpleStringProperty(value); }
public String toString() { return value; }

[Code]....

But line 05 gives me a NullPointerException, why? I do this after I've set the items of the list view by doing listView.setItems(..).