I am trying to use template in a panel. The template trigger is come from another grid item selection. But this code doesn't work....

What is wrong with my code?
Just simple copy/paste my code to run...
Anyone, would you please fix this for me?

HTML Code:
<html>
<head>
<link rel="stylesheet" type="text/css" href="extjs/resources/css/ext-all.css">
<script type="text/javascript" src="extjs/ext-all-debug.js">
</script>
<script type="text/javascript">
    Ext.onReady(function () {
    
    Ext.create('Ext.data.Store',{
        storeId: 'letterStore',
        fields: ['TitleLetter','LetterType'],
        data: {
        'items': [
            { 'TitleLetter' : 'Keterangan', 'LetterType' : 'Not My Type'},
            { 'TitleLetter' : 'Dua', 'LetterType' : 'Yes This is my Type' }
        ]
        },
        proxy: {
        type: 'memory',
        reader: {
            type: 'json',
            root: 'items'
        }
        }
    });
    
    
    var panel = Ext.create('Ext.panel.Panel',{
        title: 'Testing',
        renderTo: Ext.getBody(),
        items: [
        {
            xtype: 'gridpanel',
            title: 'Grid For TPL',
            bodyPadding: 5,
            listeners: {
            itemclick: function(selModel, record, index, options){
                
                var detailPanel = this.child('#detailPanel');
                detailPanel().update(record.data);
            }
            },
            store: Ext.data.StoreManager.lookup('letterStore'),
            width: 300,
            height: 300,
            columns: [
            {
                xtype: 'gridcolumn',
                dataIndex: 'TitleLetter',
                text: 'Judul Surat'
            },
            {
                xtype: 'gridcolumn',
                dataIndex: 'LetterType',
                text: 'Tipe Surat'
            },

            ]
        },
        {
            xtype: 'panel',
            itemId: 'detailPanel',
            title: 'Show TPL',
            tpl: ['I am trying to use TPL {TitleLetter}']
        },
        
        
        ]
    
    
    });
    
    
    
    });

</script>
</head>
<body>
</body>
</html>