title: Sencha Touch selectfield cannot display value from datastore
body: <p>I'm trying to load a store into a Select field in sencha touch 2.0 but got a strange proble:
  For following code:</p>
  
  <pre><code>        {
              xtype : 'list',
              store : 'Docbases',
              itemTpl : 'Hello {docbase}!'
          }, {
              xtype : 'selectfield',
              label : 'Docbase',
              id : 'docbase',
              store : 'Docbases',
              displayField : 'docbase',
              valueField : 'docbase',
              placeHolder : 'Select a Value'
  
          }
  </code></pre>
  
  <p>The list component can display well, while selectfield cannot display the value. When click on that selectfield, I got a console error:</p>
  
  <pre><code>Uncaught TypeError: Cannot call method 'get' of null
  </code></pre>
  
  <p>My Store is declared as:</p>
  
  <pre><code>Ext.define('FDMobileClient.store.Docbases', {
      extend : 'Ext.data.Store',
      requires : ['FDMobileClient.model.Docbase'],
      model : 'FDMobileClient.model.Docbase',
      autoLoad : true,
      proxy : {
          type : 'ajax',
          url : '/MobileInternalProject/mobile/getDocbaseList.action',
          reader : {
              type : 'json',
              root : 'docbases'
          }
      },
  
  });
  </code></pre>
  
  <p>Does anyone have any ideas what I'm doing wrong :(</p>
  
  <p>I'm appreciated all of your help. Thanks</p>
  
  <p>Long</p>