How could I create tooltip above combobox with selected record?

I try this:

Code:

listeners: {'select': function(combo, record, index){
  var quickTipText = '';        
  Ext.QuickTips.unregister(this.getEl());
  if ((record instanceof Ext.data.Record) && (record.get('field1') !== ""))
    text = record.get('field1');
  if (quickTipText) {
    Ext.QuickTips.register({
    target:  this.getEl(),
    text: quickTipText
    });
  }             
}
}

It doesn't work.



How could I create tooltip above combobox with selected record?