Loop through DataTable Row Error: Object reference not set to an instance of an object

When i am inserting record in Master Child table then exception is raising that 

Object reference not set to an instance of an object.

 int FileUpload1;
Line 70: 
Line 71:                 foreach (DataRow row in dt.Rows)
Line 72:                 {

here is my code

DataTable dt = new DataTable();
DataRow dr;
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        dt.Columns.Add("FileUpload1");
        dt.Columns.Add("_ID");

        this.BindGridview();
    }
}
private void BindGridview()
{
    DataTable dt = new DataTable();
    dt.Columns.Add("RowNumber", typeof(int));

    for (int i = 1; i < 6; i++)
    {
        dt.Rows.Add(i);
    }

    Gridview1.DataSource = dt;
    Gridview1.DataBind();
}
protected void OnUpdate(object sender, EventArgs e)
{
    System.IO.Stream fs = FileUpload2.PostedFile.InputStream;
    System.IO.BinaryReader br = new System.IO.BinaryReader(fs);
    Byte[] bytesM = br.ReadBytes((Int32)fs.Length);
    using (SqlCommand cmd = new SqlCommand("[DiplayMasterinsert]", con))
    {
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("@Name", txtname.Text);
        cmd.Parameters.AddWithValue("@Data", bytesM);
        con.Open();
        _ID = Convert.ToInt32(cmd.ExecuteScalar());
        con.Close();
        DataTable dt = (DataTable)ViewState["dt"];
        int FileUpload1;

        foreach (DataRow row in dt.Rows)
        {
            //pid = int.Parse(row["_PID"].ToString());
            FileUpload1 = int.Parse(row["FileUpload1"].ToString());
            this.InsertRows(FileUpload1);
        }
    }
}
private void InsertRows(int data)
{
    con.Open();
    using (SqlCommand cmd = new SqlCommand("[Insert_DisplayDetail]", con))
    {
        foreach (GridViewRow row in Gridview1.Rows)
        {
            FileUpload upload = (FileUpload)row.FindControl("FileUpload1");
            // if (!string.IsNullOrEmpty(customerName) && !string.IsNullOrEmpty(country) && !string.IsNullOrEmpty(upload.PostedFile.FileName))
            {
                System.IO.Stream fs = upload.PostedFile.InputStream;
                System.IO.BinaryReader br = new System.IO.BinaryReader(fs);
                Byte[] bytes = br.ReadBytes((Int32)fs.Length);

                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@ID", _ID);

                cmd.Parameters.AddWithValue("@Data", bytes);

                cmd.ExecuteNonQuery();
                con.Close();
            }
        }

        Response.Redirect(Request.Url.AbsoluteUri);
    }

}
public int _ID { get; set; }