instead of use `$_item->getProduct()->getId()`

You should use sku `$_item->getSku();` 


>     $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
>     $product = $objectManager->create('\Magento\Catalog\Model\ProductFactory'->create());
>     $product = $this->productFactory->create();
>     $product->load($product->getIdBySku($_item->getSku()));
>     $thitem = $_product->getThumbnail();

**Suggestion:**

[Use of direct product object is not good idea][1]

Also suggest to inject [\Magento\Catalog\Model\ProductFactory to your block class][2]




  [1]: https://magento.stackexchange.com/questions/117098/magento-2-to-use-or-not-to-use-the-objectmanager-directly
  [2]: https://magento.stackexchange.com/questions/111245/how-to-load-product-by-sku-in-magento-2/111248