GridView inside Master Page hides on PostBack in ASP.Net

I have Downloaded the sapme from this link

Ref:Transfer Selected Rows from one GridView to Another in Asp.net

and created the master page and web page as you suggested

HTML:

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
    CodeFile="Default.aspx.cs" Inherits="_Default" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
    <title>Add Delete Selected rows from one Gridview to another</title>
    <script type="text/javascript">
<!--
        function Check_Click(objRef) {
            //Get the Row based on checkbox
            var row = objRef.parentNode.parentNode;

            //Get the reference of GridView
            var GridView = row.parentNode;

            //Get all input elements in Gridview
            var inputList = GridView.getElementsByTagName("input");

            for (var i = 0; i < inputList.length; i++) {
                //The First element is the Header Checkbox
                var headerCheckBox = inputList[0];

                //Based on all or none checkboxes
                //are checked check/uncheck Header Checkbox
                var checked = true;
                if (inputList[i].type == "checkbox" && inputList[i] != headerCheckBox) {
                    if (!inputList[i].checked) {
                        checked = false;
                        break;
                    }
                }
            }
            headerCheckBox.checked = checked;

        }
        function checkAll(objRef) {
            var GridView = objRef.parentNode.parentNode.parentNode;
            var inputList = GridView.getElementsByTagName("input");
            for (var i = 0; i < inputList.length; i++) {
                var row = inputList[i].parentNode.parentNode;
                if (inputList[i].type == "checkbox" && objRef != inputList[i]) {
                    if (objRef.checked) {
                        inputList[i].checked = true;
                    }
                    else {
                        if (row.rowIndex % 2 == 0) {
                            row.style.backgroundColor = "#C2D69B";
                        }
                        else {
                            row.style.backgroundColor = "white";
                        }
                        inputList[i].checked = false;
                    }
                }
            }
        }
//-->
    </script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    <asp:ToolkitScriptManager ID="ScriptManager1" runat="server">
    </asp:ToolkitScriptManager>
    <div>
        <%--<asp:TabContainer ID="TabContainer1" runat="server">
            <asp:TabPanel runat="server" HeaderText="ADD" ID="TabPanel1">
                <HeaderTemplate>
                    ADD</HeaderTemplate>
                <ContentTemplate>
                    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                        <ContentTemplate>--%>
                            <asp:GridView ID="gvAll" runat="server" AutoGenerateColumns="false" Font-Names="Arial"
                                Font-Size="11pt" AlternatingRowStyle-BackColor="#C2D69B" HeaderStyle-BackColor="green"
                                AllowPaging="true" OnPageIndexChanging="OnPaging" PageSize="10">
                                <Columns>
                                    <asp:TemplateField>
                                        <HeaderTemplate>
                                            <asp:CheckBox ID="chkAll" runat="server" onclick="checkAll(this);" AutoPostBack="true"
                                                OnCheckedChanged="CheckBox_CheckChanged" />
                                        </HeaderTemplate>
                                        <ItemTemplate>
                                            <asp:CheckBox ID="chk" runat="server" onclick="Check_Click(this)" AutoPostBack="true"
                                                OnCheckedChanged="CheckBox_CheckChanged" />
                                        </ItemTemplate>
                                    </asp:TemplateField>
                                    <asp:BoundField DataField="CustomerID" HeaderText="Customer ID" HtmlEncode="false" />
                                    <asp:BoundField DataField="ContactName" HeaderText="Contact Name" HtmlEncode="false" />
                                    <asp:BoundField DataField="City" HeaderText="City" HtmlEncode="false" />
                                </Columns>
                                <AlternatingRowStyle BackColor="#C2D69B" />
                            </asp:GridView>
                            <br />
                            <asp:GridView ID="gvSelected" runat="server" AutoGenerateColumns="false" Font-Names="Arial"
                                Font-Size="11pt" AlternatingRowStyle-BackColor="#C2D69B" HeaderStyle-BackColor="green"
                                EmptyDataText="No Records Selected">
                                <Columns>
                                    <asp:BoundField DataField="CustomerID" HeaderText="Customer ID" />
                                    <asp:BoundField DataField="ContactName" HeaderText="Contact Name" />
                                    <asp:BoundField DataField="City" HeaderText="City" />
                                </Columns>
                            </asp:GridView>
                        <%--</ContentTemplate>
                    </asp:UpdatePanel>
                </ContentTemplate>
            </asp:TabPanel>
            <asp:TabPanel runat="server" HeaderText="Edit" ID="TabPanel2">
                <HeaderTemplate>
                    EDIT</HeaderTemplate>
                <ContentTemplate>
                </ContentTemplate>
            </asp:TabPanel>
        </asp:TabContainer>--%>
    </div>
</asp:Content>

C#:

protected void Page_Load(object sender, EventArgs e)
    {
        BindPrimaryGrid();
        BindSecondaryGrid();
    }

    private void BindPrimaryGrid()
    {
        string constr = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
        string query = "select CustomerID, ContactName, City from customers";
        SqlConnection con = new SqlConnection(constr);
        SqlDataAdapter sda = new SqlDataAdapter(query, con);
        DataTable dt = new DataTable();
        sda.Fill(dt);
        gvAll.DataSource = dt;
        gvAll.DataBind();
    }

    protected void OnPaging(object sender, GridViewPageEventArgs e)
    {
        GetData();
        gvAll.PageIndex = e.NewPageIndex;
        BindPrimaryGrid();
        SetData();
    }

    private void GetData()
    {
        DataTable dt;
        if (ViewState["SelectedRecords"] != null)
            dt = (DataTable)ViewState["SelectedRecords"];
        else
            dt = CreateDataTable();
        CheckBox chkAll = (CheckBox)gvAll.HeaderRow
                            .Cells[0].FindControl("chkAll");
        for (int i = 0; i < gvAll.Rows.Count; i++)
        {
            if (chkAll.Checked)
            {
                dt = AddRow(gvAll.Rows[i], dt);
            }
            else
            {
                CheckBox chk = (CheckBox)gvAll.Rows[i]
                                .Cells[0].FindControl("chk");
                if (chk.Checked)
                {
                    dt = AddRow(gvAll.Rows[i], dt);
                }
                else
                {
                    dt = RemoveRow(gvAll.Rows[i], dt);
                }
            }
        }
        ViewState["SelectedRecords"] = dt;
    }

    private void SetData()
    {
        CheckBox chkAll = (CheckBox)gvAll.HeaderRow.Cells[0].FindControl("chkAll");
        chkAll.Checked = true;
        if (ViewState["SelectedRecords"] != null)
        {
            DataTable dt = (DataTable)ViewState["SelectedRecords"];
            for (int i = 0; i < gvAll.Rows.Count; i++)
            {
                CheckBox chk = (CheckBox)gvAll.Rows[i].Cells[0].FindControl("chk");
                if (chk != null)
                {
                    DataRow[] dr = dt.Select("CustomerID = '" + gvAll.Rows[i].Cells[1].Text + "'");
                    chk.Checked = dr.Length > 0;
                    if (!chk.Checked)
                    {
                        chkAll.Checked = false;
                    }
                }
            }
        }
    }

    private DataTable CreateDataTable()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("CustomerID");
        dt.Columns.Add("ContactName");
        dt.Columns.Add("City");
        dt.AcceptChanges();
        return dt;
    }

    private DataTable AddRow(GridViewRow gvRow, DataTable dt)
    {
        DataRow[] dr = dt.Select("CustomerID = '" + gvRow.Cells[1].Text + "'");
        if (dr.Length <= 0)
        {
            dt.Rows.Add();
            dt.Rows[dt.Rows.Count - 1]["CustomerID"] = gvRow.Cells[1].Text;
            dt.Rows[dt.Rows.Count - 1]["ContactName"] = gvRow.Cells[2].Text;
            dt.Rows[dt.Rows.Count - 1]["City"] = gvRow.Cells[3].Text;
            dt.AcceptChanges();
        }
        return dt;
    }

    private DataTable RemoveRow(GridViewRow gvRow, DataTable dt)
    {
        DataRow[] dr = dt.Select("CustomerID = '" + gvRow.Cells[1].Text + "'");
        if (dr.Length > 0)
        {
            dt.Rows.Remove(dr[0]);
            dt.AcceptChanges();
        }
        return dt;
    }

    protected void CheckBox_CheckChanged(object sender, EventArgs e)
    {
        GetData();
        SetData();
        BindSecondaryGrid();
    }

    private void BindSecondaryGrid()
    {
        DataTable dt = (DataTable)ViewState["SelectedRecords"];
        gvSelected.DataSource = dt;
        gvSelected.DataBind();
    }

NameSpace:

using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Web.UI.WebControls;

Image: