Resolve #3478628 "Fatal error uncaught"

Closes #3478628

core/lib/Drupal/Core/Extension/ThemeHandler.php

@@ -67,11 +67,18 @@ public function getDefault() {
   public function listInfo() {
     if (!isset($this->list)) {
       $this->list = [];
-      $installed_themes = $this->configFactory->get('core.extension')->get('theme');
+      $installed_themes = array_keys($this->configFactory->get('core.extension')->get('theme'));
       if (!empty($installed_themes)) {
         $list = $this->themeList->getList();
-        foreach (array_keys($installed_themes) as $theme_name) {
-          $this->addTheme($list[$theme_name]);
+        foreach ($installed_themes as $theme) {
+          // Do not add installed themes that cannot be found by the
+          // extension.list.theme service. If a theme does go missing from the
+          // file system any call to ::getTheme() will result in an exception
+          // and an error being logged. Ignoring the problem here allows the
+          // theme system to fix itself while updating.
+          if (isset($list[$theme])) {
+            $this->addTheme($list[$theme]);
+          }
         }
       }
     }