Fatal error: Uncaught Error: Call to a member function isSingleStoreMode() on null in /home/esecureh/public_html/magdemo/vendor/magento/module-newsletter/view/adminhtml/templates/preview/iframeswitcher.phtml:12 Stack trace: #0 {main} thrown in /home/esecureh/public_html/magdemo/vendor/magento/module-newsletter/view/adminhtml/templates/preview/iframeswitcher.phtml on line 12