diff --git a/lib/outputlib.php b/lib/outputlib.php
index f44c12e9c18..4e3fdbe5048 100644
--- a/lib/outputlib.php
+++ b/lib/outputlib.php
@@ -963,7 +963,7 @@ class theme_config {
                 $files['plugin_'.$plugin] = $sheetfile;
             }
 
-            $subplugintypes = core_component::get_subplugins("editor_{$plugin}");
+            $subplugintypes = core_component::get_subplugins("editor_{$plugin}") ?? [];
             // Fetch sheets for any editor subplugins.
             foreach ($subplugintypes as $plugintype => $subplugins) {
                 foreach ($subplugins as $subplugin) {