nsIDOMWindow->GetContentDOMWindow problem

I use gecko 1.9.2.23

I have an error when I use the GetContentDomWindow on a
nsIWebBrowserObject

What I do before is:

I create a browser into a window. I navigate on a page using loadURI.
The page is OK

I trap a key input and start to execute some code into my chrome when
a key is pressed.

Here is the code

    nsIDOMWindow* domWindow=NULL;
	rv=mWebBrowser->GetContentDOMWindow(&domWindow);


    nsCOMPtr<nsIScriptGlobalObjectOwner>
theGlobalObjectOwner(do_GetInterface(domWindow));


But domWindow is NULL and rv=mWebBrowser-
>GetContentDOMWindow(&domWindow); returns 0x80004002.

What is wrong with y code. Do you have some sample ?

THank's a lot for your responses

philippe