code: private TreeObject createDummyModel() {
          TreeObject to1 = new TreeObject("Inbox");
          TreeObject to2 = new TreeObject("Drafts");
          TreeObject to3 = new TreeObject("Sent");
          TreeParent p1 = new TreeParent("me@this.com");
          p1.addChild(to1);
          p1.addChild(to2);
          p1.addChild(to3);
  
          TreeObject to4 = new TreeObject("Inbox");
          TreeParent p2 = new TreeParent("other@aol.com");
          p2.addChild(to4);
  
          TreeParent root = new TreeParent("");
          root.addChild(p1);
          root.addChild(p2);
          return root;
      }
docs: We will set up a dummy model to initialize tree heararchy. In real
  code, you will connect to a real model and expose its hierarchy.