Issue #3107993 - Template is not defined error on node edit / admin pages

Addresses https://www.drupal.org/project/components/issues/3107993

src/Template/ComponentsRegistry.php

@@ -4,6 +4,7 @@ namespace Drupal\components\Template;
 
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Extension\ExtensionList;
 use Drupal\Core\Extension\ModuleExtensionList;
 use Drupal\Core\Extension\ModuleHandlerInterface;
@@ -70,6 +71,13 @@ class ComponentsRegistry {
    */
   protected $fileSystem;
 
+  /**
+   * The config factory service.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
   /**
    * Constructs a new ComponentsRegistry object.
    *
@@ -85,6 +93,8 @@ class ComponentsRegistry {
    *   Cache backend for storing the components registry info.
    * @param \Drupal\Core\File\FileSystemInterface $fileSystem
    *   The file system service.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface
+   *   The config factory service.
    */
   public function __construct(
     ModuleExtensionList $moduleExtensionList,
@@ -93,6 +103,7 @@ class ComponentsRegistry {
     ThemeManagerInterface $themeManager,
     CacheBackendInterface $cache,
     FileSystemInterface $fileSystem,
+    ConfigFactoryInterface $configFactory
   ) {
     $this->moduleExtensionList = $moduleExtensionList;
     $this->themeExtensionList = $themeExtensionList;
@@ -100,6 +111,20 @@ class ComponentsRegistry {
     $this->themeManager = $themeManager;
     $this->cache = $cache;
     $this->fileSystem = $fileSystem;
+    $this->configFactory = $configFactory;
+  }
+
+  /**
+   * Retrieves a configuration object.
+   *
+   * @param string $name
+   *   The name of the configuration object to retrieve.
+   *
+   * @return \Drupal\Core\Config\ImmutableConfig
+   *   A configuration object.
+   */
+  protected function config($name) {
+    return $this->configFactory->get($name);
   }
 
   /**
@@ -117,7 +142,23 @@ class ComponentsRegistry {
       $this->load($themeName);
     }
 
-    return $this->registry[$themeName][$name] ?? NULL;
+    if (!empty($this->registry[$themeName][$name])) {
+      return $this->registry[$themeName][$name];
+    }
+
+    // If the template was not found in the active theme, and the active theme
+    // is not the same as the default theme, load the default theme and check
+    // there too. We may be dealing with the admin theme.
+    $defaultThemeName = $this->config('system.theme')->get('default');
+    if (empty($defaultThemeName) || $defaultThemeName === $themeName) {
+      return NULL;
+    }
+
+    if (!isset($this->registry[$defaultThemeName])) {
+      $this->load($defaultThemeName);
+    }
+
+    return $this->registry[$defaultThemeName][$name] ?? NULL;
   }
 
   /**