how to show Alert with TbAlert yii when delete data - yii

Question

i want to display the error with tbalert in admin page, the alert show when data is duplicate when user click delete in gridview, to check duplicate i used function beforedelete in model, but why does the alert not show in admin page ?
//code in model
public function beforeDelete()
{
$criteria = new CDbCriteria();
$criteria->condition = 'idnasabah=:idnasabah';
$criteria->params = array(':idnasabah'=>$this->idnasabah);
if (Tbpinjaman::model()->exists($criteria)) {
Yii::app()->user->setFlash(
'error',
'<strong>Ooops..!</strong> Data tidak dapat dihapus karena relasi dengan tabel pinjaman.'
);
return false;
}
else {
return parent::beforeDelete();
}
}
//code in admin page
<?php
/* #var $this TbnasabahController */
/* #var $model Tbnasabah */
$this->widget('bootstrap.widgets.TbAlert', array(
'block' => true,
'fade' => true,
'closeText' => '×', // false equals no close link
'events' => array(),
'htmlOptions' => array(),
'userComponentId' => 'user',
'alerts' => array( // configurations per alert type
// success, info, warning, error or danger
'success' => array('closeText' => '×'),
'info', // you don't need to specify full config
'warning' => array('block' => false, 'closeText' => false),
'error' => array('block' => false)
),
));
$this->widget('zii.widgets.grid.CGridView', array(
/*'type'=>'striped bordered condensed',*/
'htmlOptions'=>array('class'=>'table table-striped table-bordered table-condensed'),
'id'=>'tbregister-grid',
'filter' => $model,
'dataProvider'=>$model->search(),
'template'=>'{items}{pager}',
'columns'=>array(
array(
'header'=>'',
'headerHtmlOptions'=>array('style'=>'text-align:center;color:blue'),
'htmlOptions'=>array('style'=>'text-align:center'),
'value'=>'$this->grid->dataProvider->pagination->currentPage*$this->grid->dataProvider->pagination->pageSize + $row+1',
),