Merge forwardport of #12606 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/12606.patch (created by @Alanaktion) based on commit(s):
  1. bf611be

lib/internal/Magento/Framework/View/Design/Theme/ThemeList.php

@@ -234,7 +234,7 @@ protected function _prepareConfigurationData($themePackage)
         $media = $themeConfig->getMedia();
 
         $parentPathPieces = $themeConfig->getParentTheme();
-        if (count($parentPathPieces) == 1) {
+        if (is_array($parentPathPieces) && count($parentPathPieces) == 1) {
             $pathPieces = $pathData['theme_path_pieces'];
             array_pop($pathPieces);
             $parentPathPieces = array_merge($pathPieces, $parentPathPieces);