GwtExt EditableGridSample JavaScript Exception

I am trying out the EditableGridSample.java Demo from GwtExt.

The apllication compiles and deploys but the page is blank. I wrapped a try catch around this line

try {
        HttpProxy proxy = new HttpProxy("data/plants.xml", Connection.GET);
    } catch (Exception e) {
        Window.alert(" Exception " + e);
}

and get the error message Exception com.google.gwt.core.client.javaScriptException(TypeError): Cannot set property 'BLANK_IMAGE_URL' of undefined

Please help