Get rid of annoying "undefined index: hidden" notices

composer.json

@@ -236,6 +236,9 @@
             },
             "drupal/addtocalendar": {
               "UTC Time Adjustment": "patches/utc-time-adjustment.patch"
+            },
+            "drupal/block_permissions": {
+              "2962965": "https://www.drupal.org/files/issues/2018-09-01/block-permissions-2962965-4.patch"
             }
         }
     },

vendor/composer/installed.json

@@ -1710,6 +1710,9 @@
                     "status": "covered",
                     "message": "Covered by Drupal's security advisory policy"
                 }
+            },
+            "patches_applied": {
+                "2962965": "https://www.drupal.org/files/issues/2018-09-01/block-permissions-2962965-4.patch"
             }
         },
         "installation-source": "dist",

web/modules/block_permissions/src/BlockPermissionsPermissions.php

@@ -63,7 +63,7 @@ public function permissions() {
     // administration per theme.
     $themes = $this->themeHandler->listInfo();
     foreach ($themes as $key => $theme) {
-      if ($theme->status == 1 && $theme->info['hidden'] != 1) {
+      if (isset($theme->info['hidden']) && $theme->status == 1 && $theme->info['hidden'] != 1) {
         $permissions['administer block settings for theme ' . $key] = [
           'title' => $this->t('Administer block settings for the theme @label', ['@label' => ucfirst($theme->getName())]),
           'description' => $this->t('This permission s the administer blocks permission.'),