code: protected function renderAlertBlock()
      {
          $session = \Yii::$app->session;
          $flashes = $session->getAllFlashes();
          $alertContainerOptions = [
              'style' => 'max-width:400px'
          ];
          if (count($flashes) === 0) {
              Html::addCssStyle($alertContainerOptions, 'display:none;');
          }
          $out = Html::beginTag('div', $alertContainerOptions);
          foreach ($flashes as $type => $message) {
              if (is_array($message)) {
                  $message = implode('<br>', $message);
              }
  
              $alertWidgetOptions = [];
              $alertWidgetOptions['body'] = $message;
              $alertWidgetOptions['options'] = [
                  'class' => ['alert', 'alert-success'],
                  'style' => 'padding-left:10px;padding-right:10px;'
              ];
              $out .= "\n" . Alert::widget($alertWidgetOptions);
              $session->removeFlash($type);
          }
  
          $out .= "\n</div>";
  
          return $this->alertBlockAddon . $out;
      }
docs: Initializes and renders alert container block