bug when creating my WindowCreator

hello
i create MyWindowCreator which implements nsIWindowCreator
as explained in JavaXPCOM:Embedding Mozilla in a Java Application using 
JavaXPCOM

Mozilla mozilla = Mozilla.getInstance();
  WindowCreator creator = new WindowCreator();  // implements 
nsIWindowCreator

  nsIServiceManager serviceManager = mozilla.getServiceManager();

  nsIWindowWatcher windowWatcher = (nsIWindowWatcher) serviceManager
    .getServiceManagerByContractID(NS_WINDOWWATCHER_CONTRACTID,
      nsIWindowWatcher.NS_IWINDOWWATCHER_IID);
  windowWatcher.setWindowCreator(creator);


but when i call windowWatcher.openWindow:

Exception in thread "main" org.mozilla.xpcom.XPCOMException: The 
function "openWindow" returned an error condition  (0x80004005)
	at org.mozilla.xpcom.internal.XPCOMJavaProxy.callXPCOMMethod(Native Method)
	at 
org.mozilla.xpcom.internal.XPCOMJavaProxy.invoke(XPCOMJavaProxy.java:140)
	at $Proxy2.openWindow(Unknown Source)
	at TestWindowCreator.main(test_window_creator.java:133)

and the content of createChromeWindow is never executed

sylvain