Adding properties when mupltiple objects are selected in netbeans property sheet

Hello,

I use the Property sheet of netbeans for a professionnal project.
I have one property (which is a button) that consists on adding properties to the selected object(s). 

When 1 object is selected, it works : I can see the new properties in the property sheet without having to reselect the object.

When there are more than one object, I don't see the new properties until  I reselect the objects.

I think this is because the properties of the object are regrouped in a ProxyNode : how can I notify the ProxyNode that the new properties were added ?

Thanks