Checking for changes before selecting a different node

Hi all,
I have been following the basic CRUD tutorial on https://platform.netbeans.org/tutorials/nbm-crud.html
and I got stuck with one thing. 

When a user changes something in the Editor they put MySavable instance into an InstanceContent to enable Save action :

Code:

if (getLookup().lookup(MySavable.class) == null) {
  instanceContent.add(new MySavable());
}



This works ok. I can click Save button.
But when a user makes some changes and then selects a different node in explorer, the changes are not saved and I would like to be able to ask him to save the changes before he goes to the different node. I assumed that if I do something like this in resultChanged():

Code:

if (getLookup().lookup(MySavable.class) != null) {
  //here I inform user about changes and ask him to save
}



I would know whether something has been changed or not and I could react to that. The problem is that it is always false. I guess this is not the correct way of doing it. Can anybody explain please how to correctly check for changes before selecting a different node in explorer? 


Thanks