JUNG - adding an edge or a node dynamically in a graph

I use Java and Jung for an application with graphs. I want to add dynamically an edge to a graph while it is visible. In particular, I choose a node开发者_Go百科 from a list (JComboBox) and when I press a button an edge shall be created between that node and a given node. However, when I do that I get a Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException message. I would appreciate any help.