Problems with JSNI

[raven22k]

I'm using this class to Upload some files. I'm using smartGwt forms because they have a nice look & feel. As you can see i'm using JSNI to add the multiple attribute to the upload item. This works fine when i'm using one Form for one View. But when i have more then one Form on the view i'm getting an exception! All forms are initiated and added at the same time, when i have more then one form! 

public class GngsUploadForm extends DynamicForm {

    private UploadItem uploadItem = new UploadItem();
   
    private void initForm(final boolean multiple) {
        this.setTarget("fileUploadiFrame");
        this.setEncoding(Encoding.MULTIPART);
        this.setAction(GWT.getModuleBaseURL() +"singleFileUploadServlet");
       

        final String randomUploadItemName = Long.toString(new Date().getTime());
        uploadItem.setName(randomUploadItemName);
        uploadItem.setTitle("File: ");
        uploadItem.setRequired(true);
       
        super.setFields(uploadItem);       
       
        Scheduler.get().scheduleDeferred(new Command(){
            @Override
            public void execute(){
                if(!randomUploadItemName.equals("") && multiple) {
                    setMultiple(randomUploadItemName);
                }
            }
        });   
    }
   
    /**
     *
     */
    public GngsUploadForm(GngsUploadListener listener, boolean multiple) {
        super();
       
        initForm(multiple);
       
        if(listener != null)
            initComplete(listener);
    }

    /**
     * @param jsObj
     */
    public GngsUploadForm(JavaScriptObject jsObj, GngsUploadListener listener, boolean multiple) {
        super(jsObj);

        initForm(multiple);       
       
        if(listener != null)
            initComplete(listener);
    }
   
    @Override
    public void setFields(FormItem... fields) {
        FormItem[] newFields = new FormItem[fields.length+1];
       
        newFields[0] = uploadItem;
       
        for(int i = 1; i < fields.length+1; i++) {
            newFields[i] = fields[i-1];
        }
       
        super.setFields(newFields);
    }
   
    /*
     * if true the uploadItem is required
     * if false the uploadItem is not required
     * default: true
     * @param
     */
    public void setUploadFieldRequired(boolean uploadItemRequired) {
        uploadItem.setRequired(false);
    }
   
    private native void initComplete(GngsUploadListener listener) /*-{
       $wnd.uploadComplete = function (filename) {
               listener.@de.wwu.imi.gngs.client.helper.GngsUploadListener::uploadComplete(Ljava/lang/String;)(filename);
       };
    }-*/;
   
    private native void setMultiple(String randomUploadItemName) /*-{
        var newAttr= document.createAttribute('multiple');
        newAttr.nodeValue='multiple';
        $wnd.document.getElementsByName(randomUploadItemName)[0].setAttributeNode(newAttr);
    }-*/;   
}

Exception:

15:09:20.405 [ERROR] [gngs] Uncaught exception escaped
com.google.gwt.core.client.JavaScriptException: (TypeError): $wnd.document.getElementsByName(randomUploadItemName)[0] is undefined
    at com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:248)
    at com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:136)
    at com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:561)
....

i don't know why i'm getting this exception :( I hope someone can help me!