Let the creditmemo adminhtml controller's print action implement HttpGetActionInterface.

Author: hostep

app/code/Magento/Sales/Controller/Adminhtml/Creditmemo/AbstractCreditmemo/PrintAction.php

@@ -5,11 +5,12 @@
  */
 namespace Magento\Sales\Controller\Adminhtml\Creditmemo\AbstractCreditmemo;
 
-use Magento\Framework\App\ResponseInterface;
+use Magento\Framework\App\Action\HttpGetActionInterface;
 use Magento\Framework\App\Filesystem\DirectoryList;
+use Magento\Framework\App\ResponseInterface;
 use Magento\Sales\Api\CreditmemoRepositoryInterface;
 
-class PrintAction extends \Magento\Backend\App\Action
+class PrintAction extends \Magento\Backend\App\Action implements HttpGetActionInterface
 {
     /**
      * Authorization level of a basic admin session