Customer session undefined setData function at observer

Question

i tried to save total order at customer session for not logged in and logged in customer in sales_order_save_after observer like this:

namespace Test\Custom\Observer;

use \Psr\Log\LoggerInterface;
use Magento\Framework\Event\ObserverInterface;
class SalesOrderAfter implements ObserverInterface
{
    protected $_checkoutSession;
    protected $_customerRepositoryInterface;
    protected $_addressInterface;

  public function __construct(
  \Magento\GoogleAdwords\Helper\Data $helper,
  \Magento\Sales\Model\ResourceModel\Order\Collection $collection,
    LoggerInterface $loggerInterface,
    \Magento\Framework\App\RequestInterface $request,
    \Magento\Framework\Registry $registry,
    \Magento\Sales\Model\OrderFactory $orderFactory,
  \Magento\Customer\Model\Session $session
    )
  {
      $this->_helper = $helper;
      $this->_collection = $collection;
      $this->_logger = $loggerInterface;
            $this->orderFactory = $orderFactory;
            $this->_request = $request;
            $this->registry = $registry;
      $this->session = $session;
  }

    public function execute(\Magento\Framework\Event\Observer $observer)
    {
    $order = $observer->getEvent()->getOrder();
    $order_id = $order->getIncrementId();

    $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
    $order = $objectManager->create('Magento\Sales\Model\Order')->load($order_id);
    $this->session->getCustomersSession()->setData('total_last_order', $order->getGrandTotal()); 
    return $this;
  }
}

but i always got an error like this:

Fatal Error: 'Uncaught Error: Call to a member function setData() on null

Answer 1

You should use
$this->session->setData('total_last_order', $order->getGrandTotal());
when you use

\Magento\Customer\Model\Session $session

returns the customer session and you dont have to call getCustomerSession() separately