$alertTypes
$alertTypes : array
Alert widget renders a message from session flash. All flash messages are displayed in the sequence they were assigned using setFlash. You can set message as following:
Yii::$app->session->setFlash('error', 'This is the message');
Yii::$app->session->setFlash('success', 'This is the message');
Yii::$app->session->setFlash('info', 'This is the message');
Multiple messages could be set as follows:
Yii::$app->session->setFlash('error', ['Error 1', 'Error 2']);