Fix the issue The stock item with the \"...\" ID wasn't found after upgrading Magento

Issue: I upgraded my Magento version from 2.3.1 to 2.3.6. Everything is good except for the BSS Product Stock Alert module. I tried updating the module but the error still shows. The error breaks the website. A report ID is generated on the website.

Solution: 

The issue is caused due to that product_id is not the same with item_id after upgrading the Magento version, so when using the stockItemRepository->get(), the issue happens.

To solve it, we have modified this file app/code/Bss/ProductStockAlert/Model/Observer/ApplyProductAlertOnCollectionAfterLoadObserver.php

Please check and apply the hot-fix on the site.

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