title: Ext.MessageBox.show() doesn't show message
body: <p>I'm currently learning Sencha Touch from a book. Following the example doesn't work for me:</p>
  
  <pre><code>Ext.require('Ext.data.Store');
  Ext.require('Ext.dataview.List');
  Ext.require('Ext.MessageBox');
  new Ext.application({
    name: 'TouchStart',
    launch: function() {
      this.viewport = new Ext.Panel ({
        fullscreen: true,
        items: [{
          docked: 'top',
          xtype: 'toolbar',
          ui: 'light',
          items: [{
            text: 'Panic',
            handler: function(){
              Ext.Msg.alert('Don\'t Panic!','Keep calm');
            }
          },
          {
            text:'Greetings',
            handler: function() {
              Ext.Msg.prompt('Greetings!','What is your name?',
              function (btn,text){
                var response = new Ext.MessageBox().show({
                  title: 'Howdy',
                  msg: 'Pleased to meet you ' + text,
                });
              });
            }
          }]
        }]
      });
    }
  });
  </code></pre>
  
  <p>And in particular:</p>
  
  <pre><code>new Ext.MessageBox().show({
    title: 'Howdy',
    msg: 'Pleased to meet you ' + text,
  });
  </code></pre>
  
  <p>When I run this, only the title shows, no message is shown:</p>
  
  <p><img src="https://i.stack.imgur.com/VCv4j.png" alt="problem"></p>
  
  <p>Anyone knows why this is happening?</p>