Extjs4.0 treegrid 的加载问题 大家帮忙看看

如图的错误
下面是代码<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script src="ext-4.0.2a/ext-all.js" type="text/javascript"></script>
    <script src="ext-4.0.2a/bootstrap.js" type="text/javascript"></script>
    <link href="ext-4.0.2a/resources/css/ext-all.css" rel="stylesheet" type="text/css" />
    <script type="text/javascript">
        Ext.require([
         'Ext.data.*',
         'Ext.grid.*',
         'Ext.tree.*'
        ]);        Ext.onReady(function () {
            Ext.define('Task', {
                extend: 'Ext.data.Model',
                fields: [
            { name: 'task', type: 'string' },
            { name: 'user', type: 'string' },
            { name: 'duration', type: 'string' }
        ]
            });            var store = Ext.create('Ext.data.TreeStore', {
                model: 'Task',
                proxy: {
                    type: 'ajax',
                    url: 'treegrid.json'
                },
                folderSort: true
            });            var tree = Ext.create('Ext.tree.Panel', {
                id: 'treegrid',
                title: 'Core Team Projects',
                width: 500,
                height: 300,
                renderTo: Ext.getBody(),
                collapsible: true,
                useArrows: true,
                rootVisible: false,
                store: store,
                multiSelect: true,
                singleExpand: true,
                columns: [{
                    xtype: 'treecolumn', 
                    text: '项目列表',
                    flex: 2,
                    sortable: true,
                    dataIndex: 'task'
                }, {
                    text: '可见',
                    flex: 1,
                    sortable: true,
                    dataIndex: 'duration',
                    align: 'center',
                    renderer: function (v) {
                        if (v == 1) {
                            return "<input id='show' type='image' src='images/false.png' onclick='SetStatus(this);' />";
                        } else if (v == 2) {
                            var min = v - Math.floor(v);
                            return "<input id='show1' type='image' src='images/neglect.png' onclick='SetStatus(this);' />";
                        } else if (v == 3) {
                            return "<input id='show2' type='image' src='images/true.png' onclick='SetStatus(this);' />";
                        } else {
                            return "";
                        }
                    }                }, {
                    text: '可编辑',
                    flex: 1,
                    dataIndex: 'user',
                    sortable: true,
                    align: 'center',
                    renderer: function (v) {
                        if (v == 1) {
                            return "<input id='show' type='image' src='images/false.png' onclick='SetStatus(this);' />";
                        } else if (v == 2) {
                            return "<input id='show1' type='image' src='images/neglect.png' onclick='SetStatus(this);' />";
                        } else if (v == 3) {
                            return "<input id='show2' type='image' src='images/true.png' onclick='SetStatus(this);' />";
                        } else {
                            return "";
                        }
                    }
                }]
            });
        });        function SetStatus(obj) {
            var imgurl = obj.src;
            var type = imgurl.substring(imgurl.lastIndexOf("/") + 1, imgurl.indexOf("."));
            if (type == "true") {
                obj.src = imgurl.substring(0, imgurl.lastIndexOf("/") + 1) + "false.png";
            } else if (type == "false") {
                obj.src = imgurl.substring(0, imgurl.lastIndexOf("/") + 1) + "neglect.png";
            } else {
                obj.src = imgurl.substring(0, imgurl.lastIndexOf("/") + 1) + "true.png";
            }
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    </div>
    </form>
</body>
</html>
下面是JSON：<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script src="ext-4.0.2a/ext-all.js" type="text/javascript"></script>
    <script src="ext-4.0.2a/bootstrap.js" type="text/javascript"></script>
    <link href="ext-4.0.2a/resources/css/ext-all.css" rel="stylesheet" type="text/css" />
    <script type="text/javascript">
        Ext.require([
         'Ext.data.*',
         'Ext.grid.*',
         'Ext.tree.*'
        ]);        Ext.onReady(function () {
            Ext.define('Task', {
                extend: 'Ext.data.Model',
                fields: [
            { name: 'task', type: 'string' },
            { name: 'user', type: 'string' },
            { name: 'duration', type: 'string' }
        ]
            });            var store = Ext.create('Ext.data.TreeStore', {
                model: 'Task',
                proxy: {
                    type: 'ajax',
                    url: 'treegrid.json'
                },
                folderSort: true
            });            var tree = Ext.create('Ext.tree.Panel', {
                id: 'treegrid',
                title: 'Core Team Projects',
                width: 500,
                height: 300,
                renderTo: Ext.getBody(),
                collapsible: true,
                useArrows: true,
                rootVisible: false,
                store: store,
                multiSelect: true,
                singleExpand: true,
                columns: [{
                    xtype: 'treecolumn', 
                    text: '项目列表',
                    flex: 2,
                    sortable: true,
                    dataIndex: 'task'
                }, {
                    text: '可见',
                    flex: 1,
                    sortable: true,
                    dataIndex: 'duration',
                    align: 'center',
                    renderer: function (v) {
                        if (v == 1) {
                            return "<input id='show' type='image' src='images/false.png' onclick='SetStatus(this);' />";
                        } else if (v == 2) {
                            var min = v - Math.floor(v);
                            return "<input id='show1' type='image' src='images/neglect.png' onclick='SetStatus(this);' />";
                        } else if (v == 3) {
                            return "<input id='show2' type='image' src='images/true.png' onclick='SetStatus(this);' />";
                        } else {
                            return "";
                        }
                    }                }, {
                    text: '可编辑',
                    flex: 1,
                    dataIndex: 'user',
                    sortable: true,
                    align: 'center',
                    renderer: function (v) {
                        if (v == 1) {
                            return "<input id='show' type='image' src='images/false.png' onclick='SetStatus(this);' />";
                        } else if (v == 2) {
                            return "<input id='show1' type='image' src='images/neglect.png' onclick='SetStatus(this);' />";
                        } else if (v == 3) {
                            return "<input id='show2' type='image' src='images/true.png' onclick='SetStatus(this);' />";
                        } else {
                            return "";
                        }
                    }
                }]
            });
        });        function SetStatus(obj) {
            var imgurl = obj.src;
            var type = imgurl.substring(imgurl.lastIndexOf("/") + 1, imgurl.indexOf("."));
            if (type == "true") {
                obj.src = imgurl.substring(0, imgurl.lastIndexOf("/") + 1) + "false.png";
            } else if (type == "false") {
                obj.src = imgurl.substring(0, imgurl.lastIndexOf("/") + 1) + "neglect.png";
            } else {
                obj.src = imgurl.substring(0, imgurl.lastIndexOf("/") + 1) + "true.png";
            }
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    </div>
    </form>
</body>
</html>
能够运行成功，但是成功后一段操作节点一段时间后老是会出现如图的错误，求大侠指导！