diff -u b/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
--- b/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -50,7 +50,7 @@
     //   Stark otherwise. Since there is no low-level access to configuration
     //   currently, we only consult settings.php and fall back to Bartik
     //   otherwise, as it looks generic enough and way more user-friendly.
-    $custom_theme = variable_get('maintenance_theme', config('system.theme')->get('default'));
+    $custom_theme = variable_get('maintenance_theme', config('system.theme')->get('default')) ?: 'bartik';
   }
 
   // Ensure that system.module is loaded.
diff -u b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
--- b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
+++ b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
@@ -128,9 +128,7 @@
    */
   function testNoShortcutLink() {
     // Change to a theme that displays shortcuts.
-    config('system.theme')
-      ->set('default', 'seven')
-      ->save();
+    variable_set('theme_default', 'seven');
 
     $this->drupalGet('page-that-does-not-exist');
     $this->assertNoRaw('add-shortcut', 'Add to shortcuts link was not shown on a page not found.');
@@ -138,7 +136,9 @@
    */
   function testNoShortcutLink() {
     // Change to a theme that displays shortcuts.
-    variable_set('theme_default', 'seven');
+    config('system.theme')
+      ->set('default', 'seven')
+      ->save();
 
     $this->drupalGet('page-that-does-not-exist');
     $this->assertNoRaw('add-shortcut', 'Add to shortcuts link was not shown on a page not found.');