MAGETWO-82559: Fix app/code/Magento/Backend/Block/Media/Uploader.php getConfigJson() method #11665

 - Merge Pull Request #11665 from adrian-martinez-interactiv4/magento2:FR#FIX-BLOCK-MEDIA-UPLOADER-GETCONFIGJSON
 - Merged commits:
   1. 724f747

Author: Oleksii Korshenko

app/code/Magento/Backend/Block/Media/Uploader.php

@@ -5,6 +5,8 @@
  */
 namespace Magento\Backend\Block\Media;
 
+use Magento\Framework\App\ObjectManager;
+
 /**
  * Adminhtml media library uploader
  * @api
@@ -27,16 +29,25 @@ class Uploader extends \Magento\Backend\Block\Widget
      */
     protected $_fileSizeService;
 
+    /**
+     * @var \Magento\Framework\Json\EncoderInterface
+     */
+    protected $_jsonEncoder;
+
     /**
      * @param \Magento\Backend\Block\Template\Context $context
      * @param \Magento\Framework\File\Size $fileSize
      * @param array $data
+     * @param \Magento\Framework\Json\EncoderInterface $jsonEncoder
      */
     public function __construct(
         \Magento\Backend\Block\Template\Context $context,
         \Magento\Framework\File\Size $fileSize,
-        array $data = []
+        array $data = [],
+        \Magento\Framework\Json\EncoderInterface $jsonEncoder = null
     ) {
+        $this->_jsonEncoder = $jsonEncoder ?:
+            ObjectManager::getInstance()->get(\Magento\Framework\Json\EncoderInterface::class);
         $this->_fileSizeService = $fileSize;
         parent::__construct($context, $data);
     }
@@ -107,7 +118,7 @@ public function getJsObjectName()
      */
     public function getConfigJson()
     {
-        return $this->_coreData->jsonEncode($this->getConfig()->getData());
+        return $this->_jsonEncoder->encode($this->getConfig()->getData());
     }
 
     /**