I have column with buttons like this:

var grid = new Ext.grid.GridPanel({
            store: store,
            columns: [
        { header: 'Issue',renderer: function(val){ return '<input type="button" class="btnissue" value="Issue" />'; }, dataIndex: 'btn', width: 49 }
     ],
            renderTo: Ext.get('Issuing')
        }); 

and I want to fire click event for this buttons. if I write :

   Ext.getCmp('btnissue').on('click',function(){
    alert("message");
});

It say the button not exist. So how can I fire this event?