Update Add.php

Declare module namespace before template path

Author: mageprince

app/code/Magento/Catalog/Block/Adminhtml/Product/Attribute/Set/Toolbar/Add.php

@@ -18,7 +18,7 @@ class Add extends \Magento\Backend\Block\Template
     /**
      * @var string
      */
-    protected $_template = 'catalog/product/attribute/set/toolbar/add.phtml';
+    protected $_template = 'Magento_Catalog::catalog/product/attribute/set/toolbar/add.phtml';
 
     /**
      * @return AbstractBlock