Ext.grid.GridPanel不显示数据

我的js文件是：
  var EnrolURL = "json/Harvest/Generalize/SelectEnrol.aspx";
  var Enrolsm = new Ext.grid.CheckboxSelectionModel();  
   
  var Enrolcm = new Ext.grid.ColumnModel
  ([
  Enrolsm, 
  new Ext.grid.RowNumberer({ header: "编号", width:60 }),
  {header: "主键", dataIndex:"HarvestCode", width: 100, hidden: true },
  {header:"成果登记号",dataIndex:"HarvestRegCode", width:100,align:'center'},
  {header:"成果名称",dataIndex:"HarvestName",width:100,align:'center'},
  {header:"登记时间",dataIndex:"EnrolDate",width:100,align:'center'}
  ]);
   
  Enrolcm.defaultSortable=true;
   
  var Enrolfields =
  [
  { name: "harvestcode" },
  { name: "harvestregcode" },
  { name: "harvestname" },
  { name: "enroldate" }
  ];
   
  var EnrolStore=new Ext.data.Store
  ({
  proxy: new Ext.data.HttpProxy({ url: EnrolURL }),
  reader: new Ext.data.JsonReader({ totalProperty: "totalCount", root: "data", fields: Enrolfields })
  });
   
  EnrolStore.load({ params: { start: 0, limit: 20} });
   
  var Enrolgrid = new Ext.grid.EditorGridPanel  
  ({
  layout: "fit",
  iconCls: 'panel_icon',
  border: true,
  closable: true,
  frame: false,
  autoWidth: true,
  autoScroll: true,
  store: EnrolStore,
  sm: Enrolsm,
  cm: Enrolcm,
  viewConfig: { forceFit: true }, //重新计算所有列宽，填充grid
  clicksToEdit: 1,
  bbar: new Ext.PagingToolbar
  ({
  displayInfo: true, emptyMsg: "没有数据显示", displayMsg: "当前第{0}--{1}条数据，共{2}条",
  store: EnrolStore,
  pageSize: 20
  })
  });

后台提供的JSON数据是：
{totalCount:1,success:true,error:"",singleInfo:"",data:[{"harvestcode":"hc0001","harvestregcode":"hcEnrol0001","harvestname":"记事本","enroldate":"2010-10-13"}]}

哪位能帮帮忙呀，看看到底是哪里出错了。
O(∩_∩)O谢谢

JScript code

var Enrolgrid = new Ext.grid.EditorGridPanel  
  ({
  renderTo: Ext.getBody(),//没有渲染
  layout: "fit",
  iconCls: 'panel_icon',
  border: true,