Db session not working with flash bootstrap 4

Yii 2.0 General Discussions
1

in my view, main.php i have this

use yii\bootstrap4\Alert;
<?php 
     Alert::widget();
?>

and in my main.php

'session' => [
            'class' => 'yii\web\DbSession', 
            'sessionTable' => 'session', 
],

my seesion table look like this

CREATE TABLE `session` (
  `id` char(40) NOT NULL,
  `expire` int(11) NOT NULL,
  `data` longblob NOT NULL,
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

when i use yii\web\Session in my main.php instead of yii\web\DbSession flash works fine. But with DbSession i get this

<div id="w4" class="alert alert-dismissible" role="alert">
<button type="button" class="close" data-dismiss="alert"><span aria-hidden="true">×</span></button>
</div>

and it’s not using the proper Bootstrap 4 classes either. So i tried extending the alert class, to this

class Alert extends \yii\bootstrap4\Widget
{

    public $alertTypes = [
        'error'   => 'alert-danger',
        'danger'  => 'alert-danger',
        'success' => 'alert-success',
        'info'    => 'alert-info',
        'warning' => 'alert-warning'
    ];

    public $closeButton = [];

    public function run()
    {
        $session = Yii::$app->session;
        $flashes = $session->getAllFlashes();
        $appendClass = isset($this->options['class']) ? ' ' . $this->options['class'] : '';

        foreach ($flashes as $type => $flash) {
            if (!isset($this->alertTypes[$type])) {
                continue;
            }

            foreach ((array) $flash as $i => $message) {
                echo \yii\bootstrap4\Alert::widget([
                    'body' => $message,
                    'closeButton' => $this->closeButton,
                    'options' => array_merge($this->options, [
                        'id' => $this->getId() . '-' . $type . '-' . $i,
                        'class' => $this->alertTypes[$type] . $appendClass,
                    ]),
                ]);
            }
            $session->removeFlash($type);
        }
    }
}

but $session->getAllFlashes() gives me an empty array.

Anyone know how to fix this? thanks