ENGCOM-3136: [forwardport 2.3] fix Fatal Error when save configurable product in Magento 2.2.5 #18082 #18461 #18462

 - Merge Pull Request #18462 from thiagolima-bm/magento2:ISSUE/18082
 - Merged commits:
   1. cc16df9
   2. 7711165

Author: magento-engcom-team

app/code/Magento/ConfigurableProduct/Controller/Adminhtml/Product/Initialization/Helper/Plugin/Configurable.php

@@ -158,7 +158,7 @@ protected function getVariationMatrix()
             $configurableMatrix = json_decode($configurableMatrix, true);
 
             foreach ($configurableMatrix as $item) {
-                if ($item['newProduct']) {
+                if (isset($item['newProduct']) && $item['newProduct']) {
                     $result[$item['variationKey']] = $this->mapData($item);
 
                     if (isset($item['qty'])) {