ObjectManager cleanup - Remove usage from AdminNotification module

Fix small issues after code review

Author: ihor-sviziev

app/code/Magento/AdminNotification/Block/System/Messages.php

@@ -74,8 +74,8 @@ public function getLastCritical()
     {
         $items = array_values($this->_messages->getItems());
 
-        if (isset($items[0]) && (int)$items[0]->getSeverity() === MessageInterface::SEVERITY_CRITICAL) {
-            return $items[0];
+        if (!empty($items) && current($items)->getSeverity() === MessageInterface::SEVERITY_CRITICAL) {
+            return current($items);
         }
         return null;
     }

app/code/Magento/AdminNotification/Controller/Adminhtml/Notification/MarkAsRead.php

@@ -9,6 +9,7 @@
 use Magento\AdminNotification\Controller\Adminhtml\Notification;
 use Magento\AdminNotification\Model\NotificationService;
 use Magento\Backend\App\Action;
+use Magento\Framework\Exception\LocalizedException;
 
 class MarkAsRead extends Notification
 {
@@ -40,7 +41,7 @@ public function execute()
             try {
                 $this->notificationService->markAsRead($notificationId);
                 $this->messageManager->addSuccessMessage(__('The message has been marked as Read.'));
-            } catch (\Magento\Framework\Exception\LocalizedException $e) {
+            } catch (LocalizedException $e) {
                 $this->messageManager->addErrorMessage($e->getMessage());
             } catch (\Exception $e) {
                 $this->messageManager->addExceptionMessage(