javascript Uncaught TypeError: Cannot read property 'dom' of null

hi experts

when i use a code something like this in my javascript

var customerImg = Ext.create('Ext.Img', {
    src: 'images/customer.png',  
    width: 14,
    height: 14,    
    margin : '0 0 0 5',
    //renderTo: Ext.getBody(),
    listeners: {
        afterrender: function(c) {
            Ext.create('Ext.tip.ToolTip', {
                target: c.getEl(),
                html: 'This   represents customer delays.'
            });
        }
    }
});


{
xtype: customerImg 
},

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I end up getting error >> Uncaught TypeError: Cannot read property 'dom' of null


any idea whats wrong with the above code ?
When i comment out the above code the error goes away.

( I am using  Ext-js 4.2 )

thanks.