Fileupload fail in cluster server (JBoss)

1. access the zul page in the browser.
2. close node 1.
3. upload an image.
it will cause nullpoint execption
Problem accessing session data : class
java.lang.NullPointerException null

<zk>
<zscript>
void upload(UploadEvent event) {
org.zkoss.util.media.Media media = event.getMedia();
if (media instanceof org.zkoss.image.Image)

{ org.zkoss.zul.Image image = new org.zkoss.zul.Image(); image.setContent( (org.zkoss.image.Image) media); image.setParent(pics); }

else

{ Messagebox.show("Not an image: "+media, "Error", Messagebox.OK, Messagebox.ERROR); }

}
</zscript>
<button label="Upload" upload="true" onUpload="upload(event)"/>
<toolbarbutton label="Upload" upload="true" onUpload="upload(event)"/>
<vbox id="pics" />
</zk>