title: ExtJs4 Json TreeStore?
body: <p>I am migrating my ExtJs3 application to ExtJs4.<br>
  In ExtJs3 I had a tree grid which had a loader to load the tree data, like below:  </p>
  
  <pre><code>loader: new Ext.tree.TreeLoader({ 
              dataUrl: 'Department/DepartmentTree', 
              nestedRoot: 'Data.items'
      })
  </code></pre>
  
  <p>So I am trying to create a treeStore in ExtJs4 like below:  </p>
  
  <pre><code>   var store = Ext.create('Ext.data.TreeStore', {
          model: 'DepartmentTreeModel',
          folderSort: true,
          proxy: {
              type: 'ajax',
              url: 'Department/DepartmentTree',
              reader: {
                  type: 'json',
                  root: 'Data.items'
              }
          }
      });
  </code></pre>
  
  <p>I get an error when I call the load function on the above store <code>Cannot read property 'items' of undefined</code>.</p>
  
  <p>The JSON response looks like this:  </p>
  
  <pre><code>    {
  "Data":{
      "__type":"ListWrapperOfDepartmentTreeNodewnEzJCii:#PortalMvc.Global.Classlibrary.Model.Ui.JSONWrappers",
      "items":[{
          "ActualHeadcount":0,
          "Headcount":0,
          "Leavers":0,
          "ParentId":"~~",
          "Starters":0,
          "children":[{
              "ActualHeadcount":0,
              "Headcount":0,
              "Leavers":0,
              "ParentId":"!#",
              "Starters":0,
              "children":[{
                  "ActualHeadcount":0,
                  "Headcount":0,
                  "Leavers":0,
                  "ParentId":"*w",
                  "Starters":0,
                  "children":[{
                      "ActualHeadcount":0,
                      "Headcount":0,
                      "Leavers":0,
                      "ParentId":"*z",
                      "Starters":0,
                      "children":[],
                      "iconCls":"admin_button",
                      "id":"*{",
                      "leaf":true,
                      "text":"Parking1_subtree1"
                  }]
              }]
          }]
      }]
    }
  }
  </code></pre>