This is not a regression since it's also in NB 5.5.

Repro steps:
- create a java project with the following class

import javax.swing.JDialog;
  public class TestDialog extends JDialog {
    
    public TestDialog() {
        initComponents();
    }
        
    public void initComponents(){
        setDefaultCloseOperation(javax.swing.WindowConstants.DISPOSE_ON_CLOSE);
    }    
  }

- reverse engineer the java project
- expand the uml project down to the TestDialog class, but do not expand operation "initComponents"
- right-click on the operation "initComponents" and select "Reverse Engineer Operation" from popup menu
- when the Create New Diagram dialog comes up, click on the Cancel button to dismiss it w/o doing anything else
- expand the operation "initComponents" node
Result: an interaction node has been created. Expand the interaction node will show the nodes for a diagram but except
the diagram node. See attached screenshot.