Error: Object reference not set to an instance of an object when calculating total amount in GridView RowDataBound Event on editing Record using C# in ASP.Net

Hello,

I created a gridView which has a label in the footer that calculates the total amount of the column's values.

the problem is when I clicked on " edit link " it shows me an error tells " 

Object reference not set to an instance of an object."

in the line 

amount += Convert.ToDecimal((e.Row.FindControl("Label3") as Label).Text);

 C# codes

 protected void Page_Load(object sender, EventArgs e)
    {

  if (!IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[4]
        {
            new DataColumn("Invoice_ID", typeof(int)), new DataColumn("Invoice_Com"),
            new DataColumn("Invoice_Type"), new DataColumn("Invoice_Amount", typeof(decimal))
        });
        dt.Rows.Add(1, "Invoice_Com 1", "1", "500");
        dt.Rows.Add(2, "Invoice_Com 2", "2", "200");
        dt.Rows.Add(3, "Invoice_Com 3", "3", "1500");
        dt.Rows.Add(4, "Invoice_Com 4", "4", "789");
        GridView1.DataBind();
    }
}
 
decimal amount = 0;
protected void OnDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        amount += Convert.ToDecimal((e.Row.FindControl("Label3") as Label).Text);
    }
    if (e.Row.RowType == DataControlRowType.Footer)
    {
        (e.Row.FindControl("lblAmount") as Label).Text = amount.ToString();
    }
}

    protected void lbInsert_Click(object sender, EventArgs e)
    {
        ObjectDataSource1.InsertParameters["Invoice_Com"].DefaultValue = ((TextBox)GridView1.FooterRow.FindControl("TxtCom")).Text;
        ObjectDataSource1.InsertParameters["Invoice_Type"].DefaultValue = ((TextBox)GridView1.FooterRow.FindControl("TxtType")).Text;
        ObjectDataSource1.InsertParameters["Invoice_Amount"].DefaultValue = ((TextBox)GridView1.FooterRow.FindControl("TxtAmount")).Text;
        ObjectDataSource1.InsertParameters["Invoice_ID"].DefaultValue = ((TextBox)GridView1.FooterRow.FindControl("TxtID")).Text;
        ObjectDataSource1.Insert();

    }
}