[Solved] ASP.Net Validator Error Messages hides on PostBack

Please run this code

HTML

<asp:DropDownList ID="ddlItems" runat="server" AutoPostBack="true">
    <asp:ListItem Text="Item1" Value="Item1" />
    <asp:ListItem Text="Item1" Value="Item2" />
    <asp:ListItem Text="Item2" Value="Item3" />
</asp:DropDownList>
<asp:TextBox ID="txtProductName" runat="server" />
<asp:RequiredFieldValidator runat="server" ID="reqProductName" Display="Dynamic"
    ControlToValidate="txtProductName" ErrorMessage="Enter Product Name" ValidationGroup="registration"></asp:RequiredFieldValidator>
<asp:Button Text="Save" runat="server" OnClick="Redirect" ValidationGroup="registration" />

C#

protected void Page_Load(object sender, EventArgs e)
{        
    if (!this.IsPostBack)
    {

    }
    else
    {
        foreach (BaseValidator validator in Page.Validators)
        {
            validator.Validate();
        }
    }
}
protected void Redirect(object sender, EventArgs e)
{
    //Do some work here
}