title: Extjs Combobox ajax store doesn't render
body: <p>I have a grid in which I have column which has a combobox editor. I have the following store for the editor combobox</p>
  
  <pre><code>var combostore = Ext.create('Ext.data.Store', {
      fields: ['Value'],
      proxy: {
          type: 'ajax',
          url: '/pwbench/json/store1.json',
          reader: {
              type: 'json'
          }
      },
      autoLoad: true
  });
  </code></pre>
  
  <p>My editor config for the combobox is below</p>
  
  <pre><code>editor: {
            xtype: 'combobox',
        store: combostore,
        displayField: 'Value',
        queryMode: 'remote',
        valueField: 'Value',
        emptyText:'Select',
        autoShow: true,
        selectOnFocus:true,
        editable: false,
  }
  </code></pre>
  
  <p>The json data for the store is below</p>
  
  <pre><code>{[{"Value": "Store 1"},{"Value": "Store 1"},{"Value": "Store 1"},{"Value": "Store 1"}]}
  </code></pre>
  
  <p>The http response is fetching the json data fine, but when I click on the combobox, it doesn't show the list of options. However, if I use a store with local data, it displays the list but when I select an item and click outside of the cell of the grid, the value disappears. I need to fix both these errors. How to render the ajax json data and how to retain selected value?</p>