extjs如何只打开第2层窗口

 
在desktop桌面，建了【系统设置】打开后出现2个窗口，【系统设置】和【第2层窗口】，如图代码如下，刚接触extjs，请高手指点下，
MyDesktop.setWindow = Ext.extend(Ext.app.Module, {
    id:'set-win',
    init : function(){
        this.launcher = {
            text: '系统设置',
            iconCls:'set',
            handler : this.createWindow,
            scope: this
        }
    },    createWindow : function(){
        var desktop = this.app.getDesktop();
        var win = desktop.getWindow('set-win');
        if(!win){
            win = desktop.createWindow({
                id: 'set-win',
                title:'系统设置',
                width:740,
                height:480,
                iconCls: 'set',
                shim:false,
                animCollapse:false,
                border:false,
                constrainHeader:true,                layout: 'fit', 
                autoLoad:{url:'settings.html', scripts:true} 
                  
            });
        }
        win.show();
    }
});怎样只打开settings.html的窗口，setWindow的窗口不打开