Problem Treepanel function setRootVisible

[Arkane]

Hi,

I've tried to make a tree panel with an AsyncTree loaded from JSON
file.
I want to make the property rootVisible of TreePanel false.

But when I use this code :

$treePanel = new PhpExt_Tree_TreePanel();
$treeLoader = new PhpExt_Tree_TreeLoader();
$treeLoader->setDataUrl('tree-data.json');
$treePanel->setLoader($treeLoader);
$treePanel->setTitle('Menu');
$treePanel->setCollapsed(false);
$treePanel->setRootVisible(false);
$treePanel->setUseArrows(true);
$treePanel->setHeight(600);
$treePanel->setRoot(new PhpExt_Tree_AsyncTreeNode());

I've this error :

<b>Fatal error</b>: Property 'rootVisible' is not defined for this
object. in <b>C:\xampp\htdocs\tmf\library\PhpExt\Object.php</b> on
line <b>47</b><br />

With ExtJS (not PHP-ext) it works, there is rootVisible property.

Thanks.
Arkane