title: Json data is not populated in treeview panel
body: <p>here below is my json formate i need to show in the treeview panel in extjs6
  After loading store from server i am not able to see any records in Tree panel.
  what should be the Model view and store for this json rendering in to the Tree view panel .
  Thanks.
  here is my code : </p>
  
  <p>Model : </p>
  
  <pre><code> Ext.define('File', {
          extend: 'Ext.data.TreeModel',
          fields: [
                      {number: 'number',     type: 'string'},
          ]
      });
  </code></pre>
  
  <p>Store : </p>
  
  <pre><code>var store = Ext.create('Ext.data.TreeStore', {
          model: 'File',
          proxy: {
              type: 'ajax',
              url: 'component-tree.json',
  
              reader: {
                  type: 'json',
                  rootProperty: 'components',
  
                  leaf: true,
                  children: 'components',
                  expanded: true,
  
              },
  
          },
  
          folderSort: true,
      });
  </code></pre>
  
  <p>View : </p>
  
  <pre><code>var tree = Ext.create('Ext.tree.Panel', {
          autoLoad: true,
          collapsible: true,
          useArrows: true,
          rootVisible: true,
          store: store,
          multiSelect: true,
          width: 250,
          height: 300,
          viewConfig: {
              plugins: {
                  ptype: 'treeviewdragdrop',
                  appendOnly: true
              }
          },
          root: {
              text: "Components",
              expanded: true,
  
          },
  
  
  
          renderTo: document.body
      });
  </code></pre>
  
  <p>component-tree.json : </p>
  
  <pre><code>{
      "components": {
          "state": "RELEASED",
          "nodeType": "Component",
          "checked": true,
          "name": "PLATE - FOR CABINET - ASSEMBLY",
          "level": 0,
          "url": "http://example.com/Windchill/servlet/nexiles/tools/api/1.0/epmdocuments/OR:wt.epm.EPMDocument:4474031",
          "oid": "OR:wt.epm.EPMDocument:4474031",
          "modified": "17.04.2015",
          "filename": "21378386.asm",
          "num_children": 7,
          "number": "21378386.ASM",
          "details": "http://example.com/Windchill/app/#ptc1/tcomp/infoPage?oid=OR:wt.epm.EPMDocument:4474031&amp;u8=1",
          "components": [
              {
                  "nodeType": "Component",
                  "leaf": true,
                  "name": "PLATE - FOR CABINET - DETAIL",
                  "level": 1,
                  "url": "http://example.com/Windchill/servlet/nexiles/tools/api/1.0/epmdocuments/OR:wt.epm.EPMDocument:4474018",
                  "oid": "OR:wt.epm.EPMDocument:4474018",
                  "modified": "17.04.2011",
                  "filename": "1234.prt",
                  "state": "RELEASED",
                  "number": "21378385.PRT",
                  "details": "http://example.com/Windchill/app/#ptc1/tcomp/infoPage?oid=OR:wt.epm.EPMDocument:4474018&amp;u8=1",
                  "version": "A",
                  "checked": true,
                  "drawings": [
                      {
                          "reftype": "DRAWING",
                          "nodeType": "Drawing",
                          "leaf": true,
                          "name": "PLATE - FOR CABINET - DETAIL",
                          "url": "http://example.com/Windchill/servlet/nexiles/tools/api/1.0/epmdocuments/OR:wt.epm.EPMDocument:4474013",
                          "oid": "OR:wt.epm.EPMDocument:4474013",
                          "modified": "17.04.2015",
                          "filename": "21378385.drw",
                          "source": "representation",
                          "state": "RELEASED",
                          "number": "21378385.DRW",
                          "details": "http://example.com/Windchill/app/#ptc1/tcomp/infoPage?oid=OR:wt.epm.EPMDocument:4474013&amp;u8=1",
                          "version": "A",
                          "checked": true,
                          "id": "OR:wt.epm.EPMDocument:4474013-DRAWING",
                          "icon": "/resources/images/drawing.gif"
                      },
                      {
                          "reftype": "INTERNAL",
                          "nodeType": "Drawing",
                          "leaf": true,
                          "name": "PLATE - FOR CABINET - ASSEMBLY",
                          "url": "http://example.com/Windchill/servlet/nexiles/tools/api/1.0/epmdocuments/OR:wt.epm.EPMDocument:4474041",
                          "oid": "OR:wt.epm.EPMDocument:4474041",
                          "modified": "17.04.2015",
                          "filename": "21378386.drw",
                          "source": "representation",
                          "state": "RELEASED",
                          "number": "21378386.DRW",
                          "details": "http://example.com/Windchill/app/#ptc1/tcomp/infoPage?oid=OR:wt.epm.EPMDocument:4474041&amp;u8=1",
                          "version": "A",
                          "checked": true,
                          "id": "OR:wt.epm.EPMDocument:4474041-INTERNAL",
                          "icon": "/resources/images/drawing.gif"
                      }
                  ],
                  "icon": "/resources/images/prt_image.gif"
              }
  }
  </code></pre>