Issue #3107993: Template is not defined error

src/Template/Loader/ComponentsLoader.php

@@ -119,6 +119,11 @@ class ComponentsLoader extends FilesystemLoader {
     }
     $theme_info = $this->componentsInfo->getAllThemeInfo();
 
+    // Add theme info (theme names) to active themes - issue #3107993.
+    if (!empty($theme_info)) {
+      $active_themes = array_unique(array_merge($active_themes, array_keys($theme_info)));
+    }
+
     // Templates in namespaces should be loaded from paths in this priority:
     //
     //   1. active theme paths