bugfix Undefined constant ajax

core/UI/Traits/ContainerElements.php

@@ -52,7 +52,7 @@ trait ContainerElements
 
         $this->addElementToContainer($element, $containerName);
 
-        if (!$element->wasInitialized() && $this->getRequestValue(ajax) == 1)
+        if (!$element->wasInitialized() && $this->getRequestValue('ajax') == 1)
         {
             $element->runInitContentProcess();
         }