Error during configurable product save in Magento 2 | Notice: Undefined variable: product in /module-catalog/Controller/Adminhtml/Product/Save.php

Question

I am unable to save a configurable product. This is the error I am getting:

Notice: Undefined variable: product in /domains/devportal.flavourwarehouse.co.uk/http/vendor/magento/module-catalog/Controller/Adminhtml/Product/Save.php on line 156

Saving simple products seems to work as expected.

Working on Magento ver. 2.2.6 with products migrated from Magento ver. 1.9.

If you need more information, please let me know.

UPDATE:

In the exception logs, I got:

[2019-01-29 11:28:18] main.CRITICAL: Warning: array_filter() expects parameter 1 to be array, string given in /domains/mydomain.com/http/vendor/magento/module-configurable-product/Controller/Adminhtml/Product/Initialization/Helper/Plugin/Configurable.php on line 145 {"exception":"[object] (Exception(code: 0): Warning: array_filter() expects parameter 1 to be array, string given in /domains/mydomain.com/http/vendor/magento/module-configurable-product/Controller/Adminhtml/Product/Initialization/Helper/Plugin/Configurable.php on line 145 at /domains/mydomain.com/http/vendor/magento/framework/App/ErrorHandler.php:61)"} []

Answer 1

Seems like a Magento core bug: if the $product initialization on line 101 throws an Exception:

$product = $this->initializationHelper->initialize(
    $this->productBuilder->build($this->getRequest())
);

the $product variable will be not set in the catch section. Try to debug which kind of exception was thrown and this will give you an answer how to avoid it.

PS: exception could be found in the Magento error log.