Access HTML textarea control in code behind in ASP.Net using C#

hi

How to access textarea control invisible by using code behind. Dont suggest me to replace textbox. because i am using textarea on behalf of UNICODE Character. This UNICODE character unable 2 access by using textbox. so now i want to access the textarea control 'visible =false or true' by clicking on option button and on page load this textarea control should be invisible. On clicking the tamil option button button this control have to be visible. i had tried several coding but not got any proper result. its throwing "Object reference not set to an instance of an object" error.Then i had attched my sample coding for your reference.

<script type="text/javascript" src="../Tamil JS/angular.min.js"></script>
<script type="text/javascript" src="../Tamil JS/working.js"></script>

<table>
    <tr><td><asp:Label ID="Label2" runat="server" Text="தமிழ் "></asp:Label>
            <asp:RadioButtonList ID="rblTamilfont" AutoPostBack="true" RepeatDirection="Horizontal" OnSelectedIndexChanged="rblTamilfont_SelectedIndexChanged" runat="server">
                <asp:ListItem Text="Unicode" Selected="True" Value="0" />
                <asp:ListItem Text="Font" Value="1" />
            </asp:RadioButtonList></td>
        <td>&emsp;</td><td>&emsp;<asp:Label ID="Label3" runat="server" Text="English"></asp:Label></td></tr>
    <tr> <td>  
            
<div ng-app="myApp" ng-controller="customersCtrl">
    <textarea id="home_th_ta_box" name="home_th_ta_box" placeholder="ஏதாவது எழுதலாமே" class="home_th_ta_box txtalign" ng-keyup="($event.keyCode == 32 || $event.keyCode == 13) ? setWord() : getWord()" ng-click="getWord()"></textarea>  
    <asp:TextBox ID="TextBox2" TextMode="MultiLine" ng-keyup="($event.keyCode == 32 || $event.keyCode == 13) ? setWord() : getWord()" ng-click="getWord()" CssClass="form-control home_th_ta_box" Visible="false" placeholder="ஏதாவது எழுதலாமே" BorderColor="YellowGreen" BorderWidth="3px" Width="400" Height="150" runat="server"></asp:TextBox>
</div>

        protected void rblTamilfont_SelectedIndexChanged(object sender, EventArgs e)
        {
            HtmlTextArea txtImageupload = (HtmlTextArea)(this.FindControl("home_th_ta_box"));
            txtImageupload.Visible = false;
            HtmlTextArea bodytextarea = new HtmlTextArea();
            bodytextarea = (HtmlTextArea)(this.FindControl("home_th_ta_box"));
            if (txtImageupload.Visible = false)
            {
                TextBox3.Visible = false;
            }
            else if (txtImageupload.Visible = true)
            {
                TextBox3.Visible = true;
            }
        }