[Users] gx_mappanel in an ExtJS Tabpanel - Invalid Argument

[cr.hof at web.de]

Dear List,
in my Application I'd like to use the GeoExt Mappanel in an ExtJS  
Tabpanel.
It works fine in FF, Safari and IE8, but I get an "Invalid Argument"  
Message if I close an Ext-Tab in IE6 / IE7.

Please could someone give me a solution to this problem?

Here is an Example-Code:

Ext.onReady(function(){

     var tabs = new Ext.TabPanel({
         renderTo:'tabs',
         enableTabScroll:true,
         width:600,
         height:300
     });

     // tab generation code
     var index = 0;

     function addTab(){
         tabs.add({
             title: 'Map ' + (++index),
             items: [{
         		xtype: "gx_mappanel",
         		layers: [new OpenLayers.Layer.WMS(
             			"Global Imagery", "http://maps.opengeo.org/ 
geowebcache/service/wms",
             			{layers: "bluemarble"}
         		)],
         		width: 600,
         		height: 300,
         		zoom: 1
     		}],
             closable:true
         }).show();
     }

     new Ext.Button({
         text: 'Add Map',
         handler: addTab
     }).render(document.body, 'tabs');
});

Thanks,

Ruediger