[Solved] ASP.Net GridView Header Cell Index Error: Object reference not set to an instance of an object

I want to create a situation where on a click of a button a user account will be suspended. I have a checkbox as HeaderTemplate and checkboxes inside ItemTemplate. All these checkboxes are inside a gridview where users are displayed.

An Admin can suspend multiple rows with checkbox selection and click of button.

I tried following an article on Delete multiple rows in GridView with CheckBox selection and with confirmation in ASP.Net but in place of delete, I used update to update the users table column for suspension. But I got an error:

Object reference not set to an instance of an object.

 CheckBox chkAll = (CheckBox)UsersGrid.HeaderRow.Cells[0].FindControl("chkAll");

HTML

<asp:GridView ID="UsersGrid" runat="server" GridLines="None" AllowPaging="true" HeaderStyle-ForeColor="#224f6d" HeaderStyle-Font-Bold="true"
    AutoGenerateColumns="false" OnPageIndexChanging="OnPageIndexChanging" DataKeyNames="Uid" PageSize="7" CssClass="table" Width="100%" OnRowDataBound="OnRowDataBound" HeaderStyle-HorizontalAlign="left" RowStyle-HorizontalAlign="Left">
    <EmptyDataTemplate>
        <div>
            <asp:Label ID="labelTemp" runat="server" Text="0 Users"></asp:Label>
        </div>
    </EmptyDataTemplate>
    <Columns>
        <asp:TemplateField>
            <HeaderTemplate>
                <asp:CheckBox ID="chkAll" runat="server" onclick="checkAll(this);" />
            </HeaderTemplate>
            <ItemTemplate>
                <asp:CheckBox ID="checker" runat="server" onclick="All_Check(this);" />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:BoundField DataField="Uid" HeaderText="User ID" />
        <asp:BoundField DataField="Name" HeaderText="Organization Name" />
        <asp:BoundField DataField="email" HeaderText="Email" ReadOnly="true" />
        <asp:BoundField DataField="Role" HeaderText="User Role" ReadOnly="true" />
        <asp:BoundField DataField="IsActive" HeaderText="User Status" ReadOnly="true" />
        <asp:TemplateField HeaderText="Status">
            <ItemTemplate>
                <asp:UpdatePanel runat="server">
                    <ContentTemplate>
                        <div class="badge badge-pill badge-success" runat="server" id="Active">Active</div>
                        <div class="badge badge-pill badge-warning" runat="server" id="InActive">InActive</div>
                    </ContentTemplate>
                </asp:UpdatePanel>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>
<asp:Button ID="btnsuspend" runat="server" CssClass="btn btn-success" Font-Bold="true" Font-Size="10pt" Text="Suspended" OnClientClick="return ConfirmSuspend();" OnClick="btnsuspend_Click" />

Code

SqlCommand cmd = new SqlCommand();
SqlDataAdapter sda = new SqlDataAdapter();
DataSet ds = new DataSet();
DataTable dt = new DataTable();
SqlConnection con = new SqlConnection("Data Source=(LocalDB)\\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\\Dataregister.mdf;Integrated Security=True");
private int PageSize = 4;

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

private void AllClientsPage()
{
    SqlConnection con = new SqlConnection("Data Source=(LocalDB)\\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\\Dataregister.mdf;Integrated Security=True");
    string query = "SELECT Uid, Name, email, Role, IsActive FROM Users";
    SqlDataAdapter sda = new SqlDataAdapter(query, con);
    DataTable dt = new DataTable();
    sda.Fill(dt);
    UsersGrid.DataSource = dt;
    UsersGrid.DataBind();
}

private void GetData()
{
    ArrayList arr;
    if (ViewState["SelectedRecords"] != null)
        arr = (ArrayList)ViewState["Selectedrecords"];
    else
        arr = new ArrayList();
    CheckBox chkAll = (CheckBox)UsersGrid.HeaderRow.Cells[0].FindControl("chkAll");
    for (int i = 0; i < UsersGrid.Rows.Count; i++)
    {
        if (chkAll.Checked)
        {
            if (!arr.Contains(UsersGrid.DataKeys[i].Value))
            {
                arr.Add(UsersGrid.DataKeys[i].Value);
            }
        }
        else
        {
            CheckBox checker = (CheckBox)UsersGrid.Rows[i].Cells[0].FindControl("checker");
            if (checker.Checked)
            {
                if (!arr.Contains(UsersGrid.DataKeys[i].Value))
                {
                    arr.Add(UsersGrid.DataKeys[i].Value);
                }
            }

            else
            {
                if (arr.Contains(UsersGrid.DataKeys[i].Value))
                {
                    arr.Remove(UsersGrid.DataKeys[i].Value);
                }
            }
        }
    }
    ViewState["SelectedRecords"] = arr;
}