Merge forwardport of #11240 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/11240.patch (created by @leptoquark1) based on commit(s):
  1. 16a8050
  2. 86032de

lib/internal/Magento/Framework/View/Model/Layout/Merge.php

@@ -811,7 +811,7 @@ private function getXmlErrors($libXmlErrors)
     protected function _getPhysicalTheme(\Magento\Framework\View\Design\ThemeInterface $theme)
     {
         $result = $theme;
-        while ($result->getId() && !$result->isPhysical()) {
+        while ($result !== null && $result->getId() && !$result->isPhysical()) {
             $result = $result->getParentTheme();
         }
         if (!$result) {