Added in an isset check because if you set a flash type that is not in our…

apps/advanced/frontend/widgets/Alert.php

@@ -48,6 +48,7 @@ class Alert extends \yii\bootstrap\Widget
 		$appendCss = isset($this->options['class']) ? ' ' . $this->options['class'] : '';
 
 		foreach ($flashes as $type => $message) {
+            if (isset($this->alertTypes[$type])) {
                 /* initialize css class for each alert box */
                 $this->options['class'] = $this->alertTypes[$type] . $appendCss;
 
@@ -59,6 +60,7 @@ class Alert extends \yii\bootstrap\Widget
                     'closeButton' => $this->closeButton,
                     'options' => $this->options,
                 ]);
+            }
 
 			$session->removeFlash($type);
 		}