If it were me, i'd remove the plus button, and just load sub directories when you get tree will expand events.
So What you could do, is populate the top level node, with a Stub node under it, and show the tree with the top level node, non-expanded. When the user expands it, you will get the tree will expand event. Look at the child, and see if it's a Stub node. If it is remove it, and use apache commons to load the children, populating each one, and each one with it's own stub node.
What is a stub node? Just a node that somehow tells you that this node is a placeholder. It means that you haven't tried to load children yet. Most likely this Node class will be a special class called StubNode or something. Of course you only need to add Stub nodes to directories, not files, as you won't be expanding files.
http://www.java2s.com/Tutorial/Java/0240__Swing/TreeWillExpandListener.htm