Error Passing Value from a Modal Popup in ascx

The following code works fine if it is in an aspx form but it returns a NULL reference for innertext error if it is in an ascx form:

<link href="css/ui-lightness/jquery-ui-1.8.21.custom.css" rel="stylesheet" type="text/css" /><script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script><script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.1/jquery-ui.min.js"></script><style type="text/css">
 .ModalPopupBG
{
    background-color: #666699;
    filter: alpha(opacity=50);
    opacity: 0.7;
}

.HellowWorldPopup
{
    min-width:200px;
    min-height:150px;
    background-color:burlywood;
    border-width: 3px;
    border-style: solid;
    border-color: brown;
}
.PopupHeader
{
    background:green;
    background-color:navy;
    color:yellow;
}
</style><script>
    function onModalOk()
    {
        document.getElementById("TextBox2").innerText = document.getElementById('TextBox1').innerText;
        document.getElementById("TextBox3").innerText = $("#ddlEmp option:selected").text();
    }</script><asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><asp:TextBox ID="TextBox2" runat="server"></asp:TextBox><asp:TextBox ID="TextBox3" runat="server"></asp:TextBox><asp:button id="Button1" runat="server" text="Button" /><ajaxToolkit:modalpopupextender id="ModalPopupExtender1" runat="server"
            OnOkScript="onModalOk();"
	        cancelcontrolid="btnCancel" okcontrolid="btnOkay"
	        targetcontrolid="Button1" popupcontrolid="Panel1"
	        popupdraghandlecontrolid="PopupHeader" drag="true"
	        backgroundcssclass="ModalPopupBG"></ajaxToolkit:modalpopupextender><asp:panel id="Panel1" style="display: none" runat="server"><div class="HellowWorldPopup"><div class="PopupHeader" id="PopupHeader">People Search</div><div class="popup_Body"><p>Enter you name:<asp:textbox id="TextBox1" runat="server"  AutoCompleteType="Disabled"></asp:textbox></p><p><asp:Label ID="lblCompany" runat="server" Text="Company:"></asp:Label><asp:DropDownList ID="ddlEmp" runat="server" AutoPostBack="true"><asp:ListItem Text="Company1" Value="1"></asp:ListItem><asp:ListItem Text="Company2" Value="2"></asp:ListItem></asp:DropDownList></p></div><div class="Controls"><input id="btnOkay" type="button" value="Done" /><input id="btnCancel" type="button" value="Cancel" /></div></div></asp:panel>