I have written a code where I will get the selected text of radio button in an alert, but what I need is if user clicks on select with out selecting any radio button I would like to show an alert form the script. This is my code

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default5.aspx.cs" Inherits="Default5" %>

<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
      <ext:ResourceManager ID="ResourceManager1" runat="server" />
    <div>
        <ext:FormPanel ID="frm" runat="server">
            <Items>
                <ext:RadioGroup ID="RadioGroup1" runat="server" FieldLabel="Votre Sex ">
                    <Items>
                        <ext:Radio ID="Radio1" runat="server" BoxLabel="Male" />
                        <ext:Radio ID="Radio2" runat="server" BoxLabel="Female" />
                    </Items>
                </ext:RadioGroup>                
                <ext:Button ID="Button1" runat="server" Text="Submit">
                    <DirectEvents>
                        <Click OnEvent="Button1_Click">
                        </Click>
                    </DirectEvents>
                </ext:Button>
            </Items>
        </ext:FormPanel>
    </div>
    </form>
</body>
</html>

My code behind code 

protected void Button1_Click(object sender, DirectEventArgs e)
    {
        
        string chk = RadioGroup1.CheckedItems[0].BoxLabel.ToString();
        if (chk != null)
        {
            X.MessageBox.Alert("Click", chk).Show();
        }
        else
        {
            X.MessageBox.Alert("Click", "Select").Show();
        }
    }