I am using Magento 2 Configurable Product Matrix View ver 1.1.7 on M2.2.9.
When placing an order, I cannot checkout and got the error below
Fatal error: Uncaught Error: Call to a member function getItems() on null in /app/code/Bss/ConfigurableMatrixView/Observer/CheckoutSuccess.php:58
However, checking on the backend, I found the order was in the Order Grid.
This issue happens due to the fact that our module is not compatible with Magento 2.2.x at this time.
To fix this issue and make the module work on Magento 2.2.x, we have edited the file app/code/Bss/ConfigurableMatrixView/Observer/CheckoutSuccess.php
Please check and move this fixed file on your site and then flush the cache.
If you need any further support, please feel free to contact us.