Magento – Magento 2.2.3 – main.CRITICAL Notice: Undefined index: websites

Your error comes from this method:

/**
 * @param \Magento\Catalog\Setup\CategorySetup $categorySetup
 * @return void
 */
private function changePriceAttributeDefaultScope($categorySetup)
{
    $entityTypeId = $categorySetup->getEntityTypeId(\Magento\Catalog\Model\Product::ENTITY);
    foreach (['price', 'cost', 'special_price'] as $attributeCode) {
        $attribute = $categorySetup->getAttribute($entityTypeId, $attributeCode);
        $categorySetup->updateAttribute(
            $entityTypeId,
/*375*/     $attribute['attribute_id'],
            'is_global',
            \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_GLOBAL
        );

    }
}

The feasible issue is the absence of the attribute from the list ['price', 'cost', 'special_price'] in your system. Maybe you’ve deleted one of them?

You can find all of them in the table eav_attribute usin the query:

SELECT * FROM `eav_attribute` WHERE `attribute_code` IN ('price', 'cost', 'special_price')

You should see something similar to this:

If you want to restore your cost attribute use this query:

INSERT INTO eav_attribute (entity_type_id, attribute_code, attribute_model, backend_model, backend_type, backend_table, frontend_model, frontend_input, frontend_label, frontend_class, source_model, is_required, is_user_defined, default_value, is_unique, note) VALUES (4, 'cost', null, 'Magento\\Catalog\\Model\\Product\\Attribute\\Backend\\Price', 'decimal', null, null, 'price', 'Cost', null, null, 0, 1, null, 0, null);

You can find the fix here :

https://github.com/magento/magento2/issues/8619

It worked for me and that is to set the compiling to server side less compilation and then deleted the row (for some reason).

After that re-run php bin/magento setup:upgrade

You can find the setting in : Stores->Configuration->Advanced->Developer-> turn to default settings scope -> Front end workflow