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Issue
After upgrading to Magento 2.2.8. Cart and checkout page showing blank after adding Product to cart

Server Error

AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to a member function getFinalProduct() on null in /var/www/vhosts/mywebsite.co.uk/staging.mywebsite.co.uk/vendor/magento/module-checkout/CustomerData/DefaultItem.php:129nStack trace:n#0

/var/www/vhosts/mywebsite.co.uk/staging.mywebsite.co.uk/app/code/MGS/Mpanel/CustomerData/DefaultItem.php(81): Magento\Checkout\CustomerData\DefaultItem->getProductForThumbnail()n#1

/var/www/vhosts/mywebsite.co.uk/staging.mywebsite.co.uk/vendor/magento/module-checkout/CustomerData/AbstractItem.php(31): MGS\Mpanel\CustomerData\DefaultItem->doGetItemData()n#2

/var/www/vhosts/mywebsite.co.uk/staging.mywebsite.co.uk/vendor/magento/module-checkout/CustomerData/ItemPool.php(63): Magento\Checkout\CustomerData\AbstractItem->getItemData(Object(Magento\Quote\Model\Quote\Item\Interceptor))n#3

/var/www/vhosts/mywebsite.co.uk/staging.mywebsite.co.uk/vendor/magento/module-checkout/CustomerData/Cart.php(166): Magento\C...n', referer: https://staging.mywebsite.co.uk/baseball-cap.html

defaultitem.php

 <?php
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace MGSMpanelCustomerData;

 /**
 * Default item
 */
 class DefaultItem extends MagentoCheckoutCustomerDataDefaultItem
 
 /**
 * @var MagentoCatalogHelperImage
 */
 protected $imageHelper;

 /**
 * @var MagentoMsrpHelperData
 */
 protected $msrpHelper;

 /**
 * @var MagentoFrameworkUrlInterface
 */
 protected $urlBuilder;

 /**
 * @var MagentoCatalogHelperProductConfigurationPool
 */
 protected $configurationPool;

 /**
 * @var MagentoCheckoutHelperData
 */
 protected $checkoutHelper;

 /**
 * @var MGSMpanelHelperData
 */
 protected $panelHelper;

 /**
 * @var MGSMpanelHelperData
 */
 protected $panelImageHelper;

 /**
 * @param MagentoCatalogHelperImage $imageHelper
 * @param MagentoMsrpHelperData $msrpHelper
 * @param MagentoFrameworkUrlInterface $urlBuilder
 * @param MagentoCatalogHelperProductConfigurationPool $configurationPool
 * @param MagentoCheckoutHelperData $checkoutHelper
 * @codeCoverageIgnore
 */
 public function __construct(
 MagentoCatalogHelperImage $imageHelper,
 MagentoMsrpHelperData $msrpHelper,
 MagentoFrameworkUrlInterface $urlBuilder,
 MagentoCatalogHelperProductConfigurationPool $configurationPool,
 MagentoCheckoutHelperData $checkoutHelper,
 MGSMpanelHelperData $panelHelper,
 MGSMpanelHelperImage $panelImageHelper
 ) 
 $this->configurationPool = $configurationPool;
 $this->imageHelper = $imageHelper;
 $this->msrpHelper = $msrpHelper;
 $this->urlBuilder = $urlBuilder;
 $this->checkoutHelper = $checkoutHelper;
 $this->panelHelper = $panelHelper;
 $this->panelImageHelper = $panelImageHelper;
 

 /**
 * @inheritdoc
 */
 protected function doGetItemData()
 
 $imageSize = $this->panelHelper->getImageMinSize();
 $imageHelper = $this->panelImageHelper->init($this->getProductForThumbnail(), 'mini_cart_product_thumbnail');
 return [
 'options' => $this->getOptionList(),
 'qty' => $this->item->getQty() * 1,
 'item_id' => $this->item->getId(),
 'configure_url' => $this->getConfigureUrl(),
 'is_visible_in_site_visibility' => $this->item->getProduct()->isVisibleInSiteVisibility(),
 'product_name' => $this->item->getProduct()->getName(),
 'product_sku' => $this->item->getProduct()->getSku(),
 'product_url' => $this->getProductUrl(),
 'product_has_url' => $this->hasProductUrl(),
 'product_price' => $this->checkoutHelper->formatPrice($this->item->getCalculationPrice()),
 'product_price_value' => $this->item->getCalculationPrice(),
 'product_image' => [
 'src' => $imageHelper->getUrl(),
 'alt' => $imageHelper->getLabel(),
 'width' => $imageSize['width'],
 'height' => $imageSize['height'],
 ],
 'canApplyMsrp' => $this->msrpHelper->isShowBeforeOrderConfirm($this->item->getProduct())
 && $this->msrpHelper->isMinimalPriceLessMsrp($this->item->getProduct()),
 ];
 
 

Line 81

$imageHelper = $this->panelImageHelper->init($this->getProductForThumbnail(), 'mini_cart_product_thumbnail');

di.xml

<?xml version="1.0"?>

<type name="MGSMpanelModelMpanel">
 <arguments>
 <argument name="resource" xsi:type="object">MGSMpanelModelResourceModelMpanel</argument>
 </arguments>
</type>

<preference for="MagentoSwatchesHelperData" type="MGSMpanelHelperSwatchesData" />
<preference for="MagentoCheckoutCustomerDataDefaultItem" type="MGSMpanelCustomerDataDefaultItem" />
<preference for="MagentoConfigurableProductCustomerDataConfigurableItem" type="MGSMpanelCustomerDataDefaultItem" />
<preference for="MagentoGroupedProductCustomerDataGroupedItem" type="MGSMpanelCustomerDataDefaultItem" />
<preference for="MagentoWishlistCustomerDataWishlist" type="MGSMpanelCustomerDataWishlist" />

<preference for="MagentoFrameworkViewResultPage" type="MGSMpanelBlockFrameworkPage" />

<preference for="MagentoCmsBlockBlock" type="MGSMpanelBlockCmsBlock" />
<preference for="MagentoCmsBlockPage" type="MGSMpanelBlockCmsPage" />

<preference for="MagentoUserBlockUserEdit" type="MGSMpanelBlockAdminhtmlUserEdit" />

<preference for="MagentoCatalogBlockProductImageBuilder" type="MGSMpanelBlockProductImageBuilder" />
<preference for="MagentoCatalogHelperProductView" type="MGSMpanelHelperProductView" />
<preference for="MagentoCatalogControllerCategoryView" type="MGSMpanelControllerCategoryView" />
<preference for="MagentoCatalogModelCategory" type="MGSMpanelModelCategory" />

Can anyone please suggest a solution?