GWT & JSNI. $doc.getElementById return null

Question

I am trying to call a document Javascript'method to recover an applet (id:DesglosaApplet). In the class where I implement onModuleLoad (the main class) I put:

    public native void visualize(String city) {
       var applet = $doc.getElementById("DesglosaApplet");
       if (applet != null) {
          applet.visualizeTowers(city);
          alert("NOT NULL");
       }
       else 
          alert("NULL");
    }

It is works, applet is not null. But if I do the shame in another class (this second class called by main class) it doesn't work : I have tried $wnd.$doc , $wnd.$($doc) , $doc.applets["DesglosaApplet"] ... var applet is always null. I do not forget add in the class:

    HTML applet = new HTML();
    applet.setHTML("<applet code=\"org.jdesktop.applet.util.JNLPAppletLauncher\"" +
    "codebase=\"./\"" + "id=\"DesglosaApplet\"" +   ...  );

I am working with GWT and JSNI. I want to call an applet method in another class than that implements EntryPoint interface,

var applet = doc.getElementById("IdApplet");

Is it possible to use "$doc." in a class other than that implements Entrypoint interface?