to fix the problem you have to add some lines of code in class com.opensymphony.webwork.views.freemarker.tags.WebWorkModels

...
protected TreeNodeModel treenodeModel;
...
    public TreeNodeModel getTreenode() {
        if (treenodeModel == null) {
            treenodeModel = new TreeNodeModel(stack,req, res);
        }
        return treenodeModel;
    }