关于ext grid remove的问题

Ext.onReady(function(){

var sm = new Ext.grid.CheckboxSelectionModel();

    var cm = new Ext.grid.ColumnModel([
     new Ext.grid.RowNumberer(),
     sm,
        {header:'编号',dataIndex:'id',editor:new Ext.grid.GridEditor(new Ext.form.TextField({allowBlank:false}))},
        {header:'名称',dataIndex:'name',editor:new Ext.grid.GridEditor(new Ext.form.TextField({allowBlank:false}))},
        {header:'性别',dataIndex:'sex',editor:new Ext.grid.GridEditor(new Ext.form.TextField({allowBlank:false}))}
        //,
       // {header:'描述',dataIndex:'descn',renderer:renderDescn,sortable:true}
    ]);    var data = [
        ['1','name1','male','descn1'],
        ['2','name2','male','descn2'],
        ['3','name3','female','descn3'],
        ['4','name4','descn4']
        //,
        //['5','name5','female','descn5']
    ];    var ds = new Ext.data.Store({
        proxy: new Ext.data.HttpProxy({url:'grid.jsp'}),
        reader: new Ext.data.JsonReader({root:"root",totalProperty: 'totalProperty'}, [
            {name: 'id'},
            {name: 'name'},
            {name: 'sex'}
            //,
            //{name: 'descn'}
        ])
    });
    
    var Record = Ext.data.Record.create([
     {name:'id',type:'string'},
     {name:'name',type:'string'},
     {name:'sex',type:'string'}
     ]);
     var grid = new Ext.grid.EditorGridPanel({
        el: 'grid',
        ds: ds,
        cm: cm,
        sm:sm,
        clicksToEdit:1,
        
        tbar:new Ext.Toolbar(['-',{
            text: '添加一行',
            handler: function(){
                var p = new Record({
                    id:'',
                    name:'',
                    sex:''
                });
                grid.stopEditing();
                ds.insert(0, p);
                grid.startEditing(0, 0);
            }
         },'-',{
         text: '删除一行',
            handler: function(){
             Ext.MessageBox.show({title:'删除确认',msg:'是否删除数据',buttons:{"ok":"确定","cancel":"取消"},
             fn:function(btn){
             if(btn == 'ok' ){
             var sm = grid.getSelectionModel();
            
            
             var cell = sm.getSelections();
             alert(cell);
             ds.remove(cell);
            
            
             };
             }
             }
             );
             }
},'-'
        ]),
        
        bbar: new Ext.PagingToolbar({
        pageSize: 10,
        store: ds,
        totalProperty:'totalProperty',
        displayInfo: true,
        displayMsg: '显示第 {0} 条到 {1} 条记录，一共 {2} 条',
        emptyMsg: "没有记录"
        })
    });
    grid.render();
    ds.load({params:{start:0,limit:10}});
});
删除确认 后就报js错误 '为空或不是对象'  alert(cell); 这了 是个Objet  请高手麻烦看一下谢谢