Update Newsletter.php

Declare module namespace before template path

Author: mageprince

app/code/Magento/Customer/Block/Adminhtml/Edit/Tab/Newsletter.php

@@ -17,7 +17,7 @@ class Newsletter extends \Magento\Backend\Block\Widget\Form\Generic implements T
     /**
      * @var string
      */
-    protected $_template = 'tab/newsletter.phtml';
+    protected $_template = 'Magento_Customer::tab/newsletter.phtml';
 
     /**
      * @var \Magento\Newsletter\Model\SubscriberFactory