ติดปัญหาการ insert ขอคำแนะนำ นำดาต้าจาก gridview ไป insert - Object reference not set to an instance of an object.

private DataTable mytable;
        private string MyConnectionString = "Server=;UID=;PASSWORD=1234;Database=Machining;Max Pool Size=400;Connect Timeout=600;";

 protected void Button3_Click1(object sender, EventArgs e)
        {
            if (RadioButton2.Checked)
            {
                SqlConnection con = new SqlConnection(MyConnectionString);
                // con.Open(); // don't need the Open, the Fill will open and close the connection automatically
                SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM ADMS_Machining where datetime='" + TextBox1.Text + "'", con);
                mytable = new DataTable();
                da.Fill(mytable);
                GridView2.DataSource = mytable;
                GridView2.DataBind();


            }
            else
            {

                SqlConnection con = new SqlConnection(MyConnectionString);
                // con.Open(); // don't need the Open, the Fill will open and close the connection automatically
                SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Machining_Master where datetime='" + TextBox1.Text + "'", con);
                mytable = new DataTable();
                da.Fill(mytable);
                GridView2.DataSource = mytable;
                GridView2.DataBind();
                
            }


        }

        protected void Button4_Click(object sender, EventArgs e)
        {
             SqlConnection con = new SqlConnection(MyConnectionString);
             string SqlCmdText = "INSERT INTO Machining_Master (DailyMean,Process_Mean,Long_Term_Stdev,CPK_ESTIMATE,DAILY_MINIMUM,DAILY_MAXIMUM,Model)" + "VALUES" + "(@DailyMean, @Process_Mean, @Long_Term_Stdev,@CPK_ESTIMATE,@DAILY_MINIMUM,@DAILY_MAXIMUM,'" + this.DropDownList1.SelectedItem + "'";
    SqlCommand sc = new SqlCommand(SqlCmdText, con);
    con.Open();
    foreach (DataRow row in mytable.Rows)
    {
        sc.Parameters.Clear();
        sc.Parameters.AddWithValue("@DailyMean", row["DailyMean"]);
        sc.Parameters.AddWithValue("@Process_Mean", row["Process_Mean"]);
        sc.Parameters.AddWithValue("@Long_Term_Studev", row["Long_Term_Studev"]);
        sc.Parameters.AddWithValue("@CPK_ESTIMATE", row["CPK_ESTIMATE"]);
        sc.Parameters.AddWithValue("@DAILY_MINIMUM", row["DAILY_MINIMUM"]);
        sc.Parameters.AddWithValue("@DAILY_MAXIMUM", row["DAILY_MAXIMUM"]);
        sc.Parameters.AddWithValue("@Model", DropDownList1.SelectedItem);
        sc.ExecuteNonQuery();
    }







    con.Close();
        }



    }