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

apps/advanced/frontend/widgets/Alert.php

@@ -46,19 +46,21 @@ class Alert extends \yii\bootstrap\Widget
 		$session = \Yii::$app->getSession();
 		$flashes = $session->getAllFlashes();
 		$appendCss = isset($this->options['class']) ? ' ' . $this->options['class'] : '';
-		
+
 		foreach ($flashes as $type => $message) {
-			/* initialize css class for each alert box */
-			$this->options['class'] = $this->alertTypes[$type] . $appendCss;
+            if (isset($this->alertTypes[$type])) {
+                /* initialize css class for each alert box */
+                $this->options['class'] = $this->alertTypes[$type] . $appendCss;
 
-			/* assign unique id to each alert box */
-			$this->options['id'] = $this->getId() . '-' . $type;
+                /* assign unique id to each alert box */
+                $this->options['id'] = $this->getId() . '-' . $type;
 
-			echo \yii\bootstrap\Alert::widget([
-				'body' => $message,
-				'closeButton' => $this->closeButton,
-				'options' => $this->options,
-			]);
+                echo \yii\bootstrap\Alert::widget([
+                    'body' => $message,
+                    'closeButton' => $this->closeButton,
+                    'options' => $this->options,
+                ]);
+            }
 
 			$session->removeFlash($type);
 		}