bugfix different declaration

core/UI/MainContainer.php

@@ -30,7 +30,7 @@ class MainContainer extends Container
         $this->prepareElemnentsContainers();
     }
 
-    public function addElement($element, $containerName = null)
+    public function addElement($element = null, $containerName = null)
     {
         if (is_string($element))
         {