Fix index undefined

extension/Source/core/database/DatabaseUpdate.php

@@ -169,9 +169,11 @@ class DatabaseUpdate {
             return null;
         }
 
-        if ($_GET[ACTION_FUNCTION_UPDATE] === ACTION_FUNCTION_UPDATE_NEXT_UPDATE) {
+        $actionFunction = $_GET[ACTION_FUNCTION_UPDATE] ?? '';
+
+        if ($actionFunction === ACTION_FUNCTION_UPDATE_NEXT_UPDATE) {
             return ACTION_FUNCTION_UPDATE_NOT_PERFORMED;
-        } elseif ($_GET[ACTION_FUNCTION_UPDATE] === ACTION_FUNCTION_UPDATE_NEVER) {
+        } elseif ($actionFunction === ACTION_FUNCTION_UPDATE_NEVER) {
             return ACTION_FUNCTION_UPDATE_NEVER;
         }