Unable to get property ‘id’ of undefined or null reference

当我跟踪时,我发现在以下代码中发生错误.

var cm = new Ext.grid.ColumnModel({
    defaults: {
        sortable: true
    },columns:
      [
        {
            header: 'Result Set',dataIndex: 'result_set_name'
        },{
            header: 'Result Date',dataIndex: 'result_date',xtype: 'datecolumn',format: 'm/d/Y',dateFormat: 'c'
        },{
            header: 'Comments',dataIndex: 'comments'
        },{
           header: 'Link',dataIndex: 'link',renderer: function(value,MetaData,record,rowIndex,colIndex,store) {return '<a href="'+value+'">'+value+'</a>';}
        },]
});

但我找不到问题的根源和需要做的事情.任何帮助表示赞赏.