telerik:radcombobox id="cmboUsers" runat="server" radcomboboximageposition="Right">
  <Items>
    <telerik:RadComboBoxItem runat="server" Text="Select a user..."></telerik:RadComboBoxItem>
  </Items>
  <CollapseAnimation Type="OutQuint" Duration="200"></CollapseAnimation>
  <ExpandAnimation Type="OutQuart"></ExpandAnimation>
</telerik:radcombobox>

body onload="fnMaxWindow();fnSetFocus();">

My JavaScript code looks like:

<script language="javascript" type="text/javascript">
function fnSetFocus()
{
  var combo = $find("<%= cmboUsers.ClientID %>");
  alert("combo = " + combo);
  var inputArea = combo.get_inputDomElement();
  alert("inputArea = " + inputArea);
  inputArea.focus();
}

function fnMaxWindow() {
  window.moveTo(0,0);
  window.resizeTo(screen.width, screen.height);
}
</script>

When this runs, I get "combo = null" on the first alert, indicating that the $find code doesn't work.

Thanks in advance for the help.

 function promptCallBackFn(arg) {
                 if (arg != null) {
                    var tree = $find("<%= RadTreeView1.ClientID %>");
                    if (tree.get_selectedNode() != null) {
                        var node = tree.get_selectedNode();

                        AddNode(tree.get_selectedNode(), arg);
                    }
                }               
            }
              function AddNode(node, foldername) {

                var treeView = $find("<%= RadTreeView1.ClientID %>");
                treeView.trackChanges();
                var childNode = new Telerik.Web.UI.RadTreeNode();
                childNode.set_text(foldername);
                childNode.set_imageUrl("Img/mailfolder.gif");
                childNode.set_expandMode("Telerik.Web.UI.TreeNodeExpandMode.WebService");
                node.get_nodes().add(childNode);
                treeView.commitChanges();
               }
But When I test this,Error Occured.after I expand the Selected node first, this run well.
So I think mayby I should expanded the selected node first in my code,but it still couse error.
My questions is how to add node when we use LoadOnDemand - Webservice mode in client side.
I need a callback function to make sure that the select Node is Expanded successful and how to do this.