Cannot return null for non-nullable field SelectedCustomizableOptionValue.sort_order and Call to a member function getPriceType() on null

Author: XxXgeoXxX

app/code/Magento/QuoteGraphQl/Model/Cart/AddSimpleProductToCart.php

@@ -15,6 +15,7 @@
 use Magento\Framework\GraphQl\Exception\GraphQlNoSuchEntityException;
 use Magento\Framework\Stdlib\ArrayManager;
 use Magento\Quote\Model\Quote;
+use phpDocumentor\Reflection\Types\Mixed_;
 
 /**
  * Add simple product to cart
@@ -140,7 +141,9 @@ private function extractCustomizableOptions(array $cartItemData): array
 
         $customizableOptionsData = [];
         foreach ($customizableOptions as $customizableOption) {
-            $customizableOptionsData[$customizableOption['id']] = $customizableOption['value'];
+            $customizableOptionsData[$customizableOption['id']] = $this->convertCustomOptions(
+                $customizableOption['value']
+            );
         }
         return $customizableOptionsData;
     }
@@ -161,4 +164,17 @@ private function createBuyRequest(float $quantity, array $customOptions): DataOb
             ],
         ]);
     }
+
+    /**
+     * @param string $value
+     * @return string|array
+     */
+    private function convertCustomOptions(string $value)
+    {
+        if (substr($value, 0, 1) === "[" ||
+            substr($value, strlen($value) - 1, 1) === "]") {
+            return explode(',', substr($value, 1, -1));
+        }
+        return $value;
+    }
 }

app/code/Magento/QuoteGraphQl/etc/di.xml

@@ -26,7 +26,7 @@
                 <item name="area" xsi:type="string">Magento\QuoteGraphQl\Model\CartItem\DataProvider\CustomizableOptionValue\Text</item>
                 <item name="drop_down" xsi:type="string">Magento\QuoteGraphQl\Model\CartItem\DataProvider\CustomizableOptionValue\Dropdown</item>
                 <item name="radio" xsi:type="string">Magento\QuoteGraphQl\Model\CartItem\DataProvider\CustomizableOptionValue\Dropdown</item>
-                <item name="checkbox" xsi:type="string">Magento\QuoteGraphQl\Model\CartItem\DataProvider\CustomizableOptionValue\Dropdown</item>
+                <item name="checkbox" xsi:type="string">Magento\QuoteGraphQl\Model\CartItem\DataProvider\CustomizableOptionValue\Multiple</item>
                 <item name="multiple" xsi:type="string">Magento\QuoteGraphQl\Model\CartItem\DataProvider\CustomizableOptionValue\Multiple</item>
             </argument>
         </arguments>