validation of combobox that include treeview

<td>
                   <setsRC:SRadComboBox ID="rcbOrganizationUnitWithTemplate" runat="server" TabIndex="3"
                       OnClientDropDownOpened="OnClientDropDownOpenedHandler" Height="450" Width="350"
                       ClientIDMode="Static" InitialValue="<%$ Resources:PayrollResources, SelectValue %>">
                       <ItemTemplate>
                           <div id="divTreeInCombo">
                               <setsRC:SRadTreeView ID="STreeViewInCombo" runat="server" Width="100%" Height="450px"
                                   OnClientNodeClicked="nodeClicking" ClientIDMode="Static">
                                   <DataBindings>
                                       <telerik:RadTreeNodeBinding Expanded="True" />
                                   </DataBindings>
                               </setsRC:SRadTreeView>
                           </div>
                       </ItemTemplate>
                       <Items>
                           <setsRC:SRadComboBoxItem Text="" />
                       </Items>
                   </setsRC:SRadComboBox>
               </td>
               <setsTB:SValidRequiredFieldValidator ID="rfvOrganizationUnit" runat="server" ControlToValidate="rcbOrganizationUnitWithTemplate"
                   CssClass="MessageLabel" Display="Dynamic" ErrorMessage="<%$ Resources: , rfvOrganizationUnit.ErrorMessage %>"
                   Text="<%$ Resources:PayrollResources, WarningIcon %>" InitialValue="<%$ Resources:PayrollResources, SelectValue %>"
                   ValidationGroup="vgAccountingDesign" />