Extjs如何使用GMapPanel在地图上增加标记

以下是extjs使用gmappanel，我想点击按钮时，在地图时增加个标记，可是没反应谁帮我看看代码Ext.onReady(function() {
       var  mappanel = new Ext.Panel({
                layout : 'border',
                width:900,
                height:500,
                items: {
                    xtype: 'gmappanel',
                    id:'map2',
                    region: 'center',
                    zoomLevel: 14,
                    gmapType: 'map',
                    mapConfOpts: ['enableScrollWheelZoom','enableDoubleClickZoom','enableDragging'],
                    mapControls: ['GSmallMapControl','GMapTypeControl','NonExistantControl'],
                    setCenter: {
                       lat: 31.22, lng: 121.47,
                        er: {title: '井盖1'}
                    },
                    ers: [{
                        lat: 40.22,//北京纬度
                        lng: 116.2,//北京经度
                        er: {title: 'Boston Museum of Fine Arts'},
                        listeners: {
                            click: function(e){
                                Ext.Msg.alert('Its fine', 'and its art.');
                            }
                        }
                    }]
                    
                },
                tbar:[{
                     text:"d",
                     listeners:{
                     click:function(){
                       
                       var point = new GLatLng(31.22,121.45);//我想点击按钮时在地图上增加个标记
                        var map=Ext.get("map2");////                      
                        map.addMarkers(point);///
                     
                     }}}]
            });
        mappanel.render('ggmap');
});