I have been practicing to make a really simple grid example on JSFiddle, I've stuck in this problem. 

I have 1 model, 1 store, and 1 grid object. I think I put everything properly into appropriate spaces, but all I got from the result was 'Cannot read property "items" of undefined', can anyone please explain why this happens to me, please?

```js
var model = Ext.create('Ext.data.Model', {
  fields: [
    {name: 'name', type: 'string'},
    {name: 'value', type: 'float'}
  ]
});

var store = Ext.create('Ext.data.Store', {
  model: model,
  data: [
    {name: 'a', value: 1.2},
    {name: 'b', value: 2.3},
    {name: 'c', value: 3.4},
    {name: 'd', value: 4.5},
  ]
});

Ext.create('Ext.grid.Panel', {
  width: 500,
  height: 500,
  store: store,
  columns: [
    {text: 'header', dataIndex: 'name'},
    {text: 'header2', dataIndex: 'value'}
  ],
  renderTo: Ext.getBody()
})
```