Added _customerCollection property as public

Author: maxalmonte14

app/code/Magento/CustomerImportExport/Model/ResourceModel/Import/Customer/Storage.php

@@ -48,6 +48,11 @@ class Storage
      */
     private $customerCollectionFactory;
 
+    /**
+     * @var CustomerCollection
+     */
+    public $_customerCollection;
+
     /**
      * @param CustomerCollectionFactory $collectionFactory
      * @param CollectionByPagesIteratorFactory $colIteratorFactory
@@ -58,6 +63,9 @@ public function __construct(
         CollectionByPagesIteratorFactory $colIteratorFactory,
         array $data = []
     ) {
+        $this->_customerCollection = isset(
+            $data['customer_collection']
+        ) ? $data['customer_collection'] : $collectionFactory->create();
         $this->_pageSize = isset($data['page_size']) ? $data['page_size'] : 0;
         $this->_byPagesIterator = isset(
             $data['collection_by_pages_iterator']