diff --git a/core/lib/Drupal/Core/Controller/ControllerBase.php b/core/lib/Drupal/Core/Controller/ControllerBase.php
index 51c9501..3af41f5 100644
--- a/core/lib/Drupal/Core/Controller/ControllerBase.php
+++ b/core/lib/Drupal/Core/Controller/ControllerBase.php
@@ -110,11 +110,6 @@
   protected $formBuilder;
 
   /**
-   * @var
-   */
-  protected $themeHandler;
-
-  /**
    * {@inheritdoc}
    */
   public static function create(ContainerInterface $container) {
diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index a0110b3..207959a 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -461,6 +461,9 @@ protected function resetSystem() {
    */
   public function getName($theme) {
     $themes = $this->listInfo();
+    if (!isset($themes[$theme])) {
+      throw new \InvalidArgumentException(String::format('Requested the name of a non-existing theme @theme', array('@theme' => $theme)));
+    }
     return String::checkPlain($themes[$theme]->info['name']);
   }
 
diff --git a/core/modules/block/lib/Drupal/block/Controller/BlockController.php b/core/modules/block/lib/Drupal/block/Controller/BlockController.php
index 41401f1..a4e7952 100644
--- a/core/modules/block/lib/Drupal/block/Controller/BlockController.php
+++ b/core/modules/block/lib/Drupal/block/Controller/BlockController.php
@@ -9,6 +9,9 @@
 
 use Drupal\Component\Utility\String;
 use Drupal\Core\Controller\ControllerBase;
+use Drupal\Core\Extension\ThemeHandler;
+use Drupal\Core\Extension\ThemeHandlerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\Request;
 
 /**
@@ -17,6 +20,32 @@
 class BlockController extends ControllerBase {
 
   /**
+   * The theme handler.
+   *
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
+   */
+  protected $themeHandler;
+
+  /**
+   * Constructs a new BlockController instance.
+   *
+   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
+   *   The theme handler.
+   */
+  public function __construct(ThemeHandlerInterface $theme_handler) {
+    $this->themeHandler = $theme_handler;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('theme_handler')
+    );
+  }
+
+  /**
    * Returns a block theme demo page.
    *
    * @param string $theme
@@ -26,9 +55,8 @@ class BlockController extends ControllerBase {
    *   A render array containing the CSS and title for the block region demo.
    */
   public function demo($theme) {
-    $themes = list_themes();
     return array(
-      '#title' => String::checkPlain($themes[$theme]->info['name']),
+      '#title' => String::checkPlain($this->themeHandler->getName($theme)),
       '#attached' => array(
         'js' => array(
           array(
@@ -47,18 +75,4 @@ public function demo($theme) {
     );
   }
 
-  /**
-   * Get the title.
-   *
-   * @param string $theme
-   *   The configured theme.
-   *
-   * @return string
-   *   The human readable name of the theme.
-   */
-  public function getTitle($theme) {
-    $themes = $this->themeHandler->listInfo();
-    return String::checkPlain($themes[$theme]->info['name']);
-  }
-
 }