[Solved] ASP.Net Error: The GridView fired event PageIndexChanging which wasn't handled

I use this Reference For Paging In GridView. But Still Have Errors.

Server Error in '/' Application.
The GridView 'gvInvoices' fired event PageIndexChanging which wasn't handled.

 Paging in ASP.Net GridView Example

namespace _5M_Solution
{
  public partial class Invoices : System.Web.UI.Page
  {
    protected void Page_Load(object sender, EventArgs e)
    {

      System.Web.UI.HtmlControls.HtmlGenericControl li = (System.Web.UI.HtmlControls.HtmlGenericControl)this.Page.Master.FindControl("Main_lnkmnu_Harqat_Fawatir");
      li.Attributes.Add("class", "menu single-menu active");

      if (!this.IsPostBack)
      {
        this.BindDropDownList();
        this.BindData(string.Empty, string.Empty);
      }
    }

    protected void OnSelectedIndexChanged(object sender, EventArgs e)
    {
      if (ddlBranchNumbers.SelectedIndex == 0)
      {
        this.BindData(string.Empty, string.Empty);
        ddlMachineNumbers.Visible = false;
      }
      if (ddlBranchNumbers.SelectedIndex > 0)
      {        
        ddlMachineNumbers.Visible = false;
        this.BindData(ddlBranchNumbers.SelectedValue, string.Empty);
      }
    }

    protected void OnPageIndexChanging(object sender, GridViewPageEventArgs e)
    {
      gvInvoices.PageIndex = e.NewPageIndex;
      this.BindData(string.Empty, string.Empty);
    }
}    

<%@ Page Title="" Language="C#" MasterPageFile="~/Site1.Master" AutoEventWireup="true" CodeBehind="Invoices.aspx.cs" Inherits="_5M_Solution.Invoices" %>


<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">    
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">

    <div class="layout-px-spacing">
        <div class="page-header">            
        </div>

        <div class="row layout-top-spacing">            

            <div class="col-xl-12 col-lg-12 col-md-12 col-12 layout-spacing">

                <div class="widget widget-content-area br-4">
                    <div class="widget-one">
                        <div class="row mt-3">
                            <div class="col-12">
                                <asp:GridView runat="server" CssClass="table table-striped table-bordered"
                                    AutoGenerateColumns="false" AllowPaging="true" PageSize="14"
                                    Width="100%" ID="gvInvoices">
                                    <Columns>
                                        <asp:BoundField DataField="branch_num" HtmlEncode="false" HeaderText="رقم المحطة<br/>Branch No" />
                                        <asp:BoundField DataField="_date" HtmlEncode="false" DataFormatString="{0:dd/MM/yyyy}" HeaderText="التاريخ<br/>Date" />
                                        <asp:BoundField DataField="invoice_number" HtmlEncode="false" HeaderText="رقم الفاتورة<br/>Invoice No" />
                                        <asp:BoundField DataField="item" HtmlEncode="false" HeaderText="الصنف<br/>Gasoline Type" />
                                        <asp:BoundField DataField="machineID" Visible="false" HtmlEncode="false" HeaderText="رقم المكينة<br/>Machine ID" />
                                        <asp:BoundField DataField="payment_type" HtmlEncode="false" HeaderText="طريقة الدفع<br/>Payment Type" />
                                        <asp:BoundField DataField="price" HtmlEncode="false" HeaderText="السعر<br/>Price" />
                                        <asp:BoundField DataField="qty" HtmlEncode="false" HeaderText="الكمية<br/>Qty" />
                                        <asp:BoundField DataField="total" HtmlEncode="false" HeaderText="الإجمالي<br/>Total Amount" />
                                        <asp:BoundField DataField="vat_amount" HtmlEncode="false" HeaderText="الضريبة<br/>VAT Amount" />
                                        <asp:BoundField DataField="userID" HtmlEncode="false" HeaderText="المستخدم<br/>User" />                                       
                                        <asp:BoundField DataField="buyer_nameAR" HtmlEncode="false" HeaderText="اسم العميل<br/>Customer Name" />
                                        <asp:BoundField DataField="coupon" HtmlEncode="false" HeaderText="رقم الكوبون<br/>Coupon No" />                                        
                                    </Columns>
                                    <PagerStyle HorizontalAlign="Center" CssClass="GridPager" />
                                    <HeaderStyle HorizontalAlign="Center" />                                   
                                </asp:GridView>
                            </div>
                        </div>
                        <div class="row">
                            <div class="col-md-12 text-right">
                                <asp:Label ID="lblTotal" Text="إجمالي عدد السجلات" runat="server" />
                                <asp:Label ID="lblTotalRecords" CssClass="badge-chip badge-danger mt-1 mb-1 ml-1" runat="server" />
                            </div>
                        </div>
                    </div>
                </div>
            </div>
        </div>
    </div>  
</asp:Content>