modal popup extender

I don't understand why my popup is not staying open. The instant I click the button, the popup appears for a split second and closes immediately. I have enabled popup blockers in my browser. I have tried this with both firefox and the edge browser. Both of the shut the popup immediately. What is the reason?

<%@ Page Title="" Language="VB" MasterPageFile="~/templates/default.master" AutoEventWireup="false" CodeFile="Default3.aspx.vb" Inherits="Default3" %><%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %><asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server"><style type="text/css">
        .Background {
            background-color: Black;
            filter: alpha(opacity=90);
            opacity: 0.8;
        }

        .Popup {
            background-color: #FFFFFF;
            border-width: 3px;
            border-style: solid;
            border-color: black;
            padding-top: 10px;
            padding-left: 10px;
            width: 400px;
            height: 350px;
        }

        .lbl {
            font-size: 16px;
            font-style: italic;
            font-weight: bold;
        }
    </style></asp:Content><asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server"></asp:Content><asp:Content ID="Content3" ContentPlaceHolderID="ContentPlaceHolder2" runat="Server"><asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><asp:Button ID="Button1" runat="server" Text="Fill Form in Popup" /><!-- ModalPopupExtender --><cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panl1" TargetControlID="Button1"
        CancelControlID="Button2" BackgroundCssClass="Background"></cc1:ModalPopupExtender><asp:Panel ID="Panl1" runat="server" CssClass="Popup" align="center" Style="display: none"><table><tr><td><asp:Label runat="server" CssClass="lbl" Text="First Name"></asp:Label></td><td><asp:TextBox runat="server" Font-Size="14px"></asp:TextBox></td></tr><tr><td><asp:Label ID="Label1" runat="server" CssClass="lbl" Text="Middle Name"></asp:Label></td><td><asp:TextBox ID="TextBox1" runat="server" Font-Size="14px"></asp:TextBox></td></tr><tr><td><asp:Label ID="Label2" runat="server" CssClass="lbl" Text="Last Name"></asp:Label></td><td><asp:TextBox ID="TextBox2" runat="server" Font-Size="14px"></asp:TextBox></td></tr><tr><td><asp:Label ID="Label3" runat="server" CssClass="lbl" Text="Gender"></asp:Label></td><td><asp:TextBox ID="TextBox3" runat="server" Font-Size="14px"></asp:TextBox></td></tr></table><asp:Button ID="Button2" runat="server" Text="Cancel" /><br /></asp:Panel></asp:Content>