Merge pull request magento#1480 from magento-nord/MAGETWO-72415

Fixed issues:
- MAGETWO-72415: Unsafe call to extension attributes in downloadable CartItemProcessor

Author: dvoskoboinikov

app/code/Magento/Downloadable/Model/Quote/Item/CartItemProcessor.php

@@ -63,6 +63,7 @@ public function __construct(
     public function convertToBuyRequest(CartItemInterface $cartItem)
     {
         if ($cartItem->getProductOption()
+            && $cartItem->getProductOption()->getExtensionAttributes()
             && $cartItem->getProductOption()->getExtensionAttributes()->getDownloadableOption()
         ) {
             $downloadableLinks = $cartItem->getProductOption()->getExtensionAttributes()->getDownloadableOption()

app/code/Magento/Downloadable/Test/Unit/Model/Quote/Item/CartItemProcessorTest.php

@@ -111,6 +111,20 @@ public function testConvertToBuyRequest()
         $this->assertEquals($buyRequestMock, $this->model->convertToBuyRequest($cartItemMock));
     }
 
+    public function testConvertToBuyRequestWithoutExtensionAttributes()
+    {
+        $cartItemMock = $this->createPartialMock(
+            \Magento\Quote\Model\Quote\Item::class,
+            ['getProductOption', 'setProductOption', 'getOptionByCode', 'getQty']
+        );
+        $productOptionMock = $this->createMock(\Magento\Quote\Api\Data\ProductOptionInterface::class);
+
+        $cartItemMock->expects($this->any())->method('getProductOption')->willReturn($productOptionMock);
+        $productOptionMock->expects($this->atLeastOnce())->method('getExtensionAttributes')->willReturn(null);
+
+        $this->assertNull($this->model->convertToBuyRequest($cartItemMock));
+    }
+
     public function testProcessProductOptions()
     {
         $downloadableLinks = [1, 2];