java.lang.NullPointerException when calling closeAllWindows

    try{
        //Gets start page
        webClient = new WebClient(BrowserVersion.FIREFOX_3);
        webClient.setThrowExceptionOnScriptError(false);
        webClient.setJavaScriptTimeout(10000); //Javascript timeout in ms
        page1 = webClient.getPage(proxy.url);

        boolean foundForm = false;
        HtmlSubmitInput button = null;
        HtmlTextInput text = null;

        //Searches for appropriate form to fill: should have a 'submit' and a 'text' field and should redirect to current website
        for (int formCount=0; (formCount<page1.getForms().size()) && (!foundForm); ++formCount){
            HtmlForm form = page1.getForms().get(formCount);
            button = null;
            text = null;

            //Checks elements in form and identifies a submit button and a text field
            for (int elementCount=0; (elementCount<form.getElementsByTagName("input").size()) && ((text == null) || (button == null)); ++elementCount){
                if ("submit".equals(form.getElementsByTagName("input").get(elementCount).getAttribute("type").toLowerCase())) button = (HtmlSubmitInput) form.getElementsByTagName("input").get(elementCount);
                if ("text".equals(form.getElementsByTagName("input").get(elementCount).getAttribute("type").toLowerCase())) text = (HtmlTextInput) form.getElementsByTagName("input").get(elementCount);
            }
            foundForm = (text != null) && (button != null);

            //Checks that it is not a foreign form (ie, that it redirects on same website)
            if (foundForm){
                foundForm = (form.getActionAttribute().toLowerCase().startsWith("http") == false) &&
                            (form.getActionAttribute().toLowerCase().equals("#") == false);
            }
        }

        //If appropriate form found
        if ((text != null) && (button != null)){
            //Fills text element with target url
            text.setValueAttribute(urlVO.getUrl());

            //Clicks on submit
            page2 = button.click();

            //Gets result
            result = page2.getWebResponse().getContentAsString();

            //Checks valid rss if needed
            if (checkRss){
                if ((! result.toLowerCase().contains("<rss")) || (! result.toLowerCase().contains("</rss"))){
                    result = null;
                    exceptStr = "Invalid RSS content";
                }
            }
        }
        else exceptStr = "No appropriate form found";
        text = null;
        button = null;
    }
    catch (Exception ex){
        exceptStr = ex.toString();
    }

    //Cleans memory
    try{
        if (page2 != null) page2.cleanUp();
        if (page1 != null) page1.cleanUp();
        if ((webClient != null) && (webClient.getTopLevelWindows().size() > 0)) webClient.closeAllWindows(); //<== EXCEPTION OCCURS HERE
        webClient = null;
    }
    catch (Exception ex){ throw new RuntimeException("Error while cleaning memory.", ex); }

    //Updates proxy DB with success/failure data
    if (result == null){
        ++proxy.reqFailed;
        ++proxy.reqTotal;
        proxy.reqLastException = exceptStr;
    }
    else{
        ++proxy.reqTotal;
        proxy.reqLastSuccess = new Date();
    }
    ProxyListDAO.updateProxy(proxy);

    System.out.println("Done");

    return result;
}