AjaxControlToolkit.dll Version difference between 4.5.7.1002 & 4.1.51116.0 with Combobox.FindControl()

Hi,
My aspx code for Combo box as
<asp:ComboBox ID="EmpNameComboBox"  runat="server"  AutoCompleteMode="SuggestAppend"  CaseSensitive="false" AppendDataBoundItems="true"  OnSelectedIndexChanged="EmpNameComboBox_SelectedIndexChanged" CausesValidation="false"  AutoPostBack="True" > </asp:ComboBox>
and code behind as

protected void EmpNameComboBox_SelectedIndexChanged(objectsender, EventArgse)
{
TextBox selectedEmpTxtBox =
EmpNameComboBox.FindControl("TextBox") asTextBox;
}
Now the above code behind retrieving selected EmpNameComboBox  value into selectedEmpTxtBox with AjaxControlToolKit.dll V4.1.51116.0  but the same code returning NULL(Not able to ...