Check if child is null before adding it to variants array. See #736

Author: minesh93

app/code/Magento/ConfigurableProductGraphQl/Model/Resolver/ConfigurableVariant.php

@@ -95,7 +95,9 @@ public function resolve(Field $field, $context, ResolveInfo $info, array $value
             $variants = [];
             /** @var Product $child */
             foreach ($children as $key => $child) {
-                $variants[$key] = ['sku' => $child['sku'], 'product' => $child, 'options' => $options];
+                if($child != null) {
+                    $variants[$key] = ['sku' => $child['sku'], 'product' => $child, 'options' => $options];
+                }
             }
 
             return $variants;