Cannot read property 'offsetWidth' of null

[coder]

In my code I use the following technique to download files

Public void onSuccess(Object result) {            

                String reportName = (String)result;                

                String url = GWT.getModuleBaseURL() + "ReportUploadHandler" + "?rname=" + reportName;

                Frame frame = new Frame(url);

                frame.setVisible(false);                

                RootPanel.get().add(frame);                

}

It seems to work just fine however occasionally I see the following message (sometimes repeated several times)

Cannot read property 'offsetWidth' of null in the console of the Chrome browser. I have never seen it in Fire Fox.

Any ideas?