รบกวนช่วยทีครับ INSERT ข้อมูลและเป็นแบบนี้อ่าครับ ExecuteNonQuery: Connection property has not been initialized รบกวนด้วยครับ จนปัญญา

private void frmCourse_Load(object sender, EventArgs e)
        {
            
            string strConn;
            strConn = DBConnString.strConn;
            Conn = new SqlConnection();

            if (Conn.State == ConnectionState.Open)
            {
                Conn.Close();
            }

            Conn.ConnectionString = strConn;
            Conn.Open();
       
        }

        private void FormatDgvCourse()
        {
            if (dgvCourse.RowCount >= 0)
            {
                dgvCourse.Columns[1].Width = 20;
                dgvCourse.Columns[1].Width = 100;
                dgvCourse.Columns[2].Width = 80;
                dgvCourse.Columns[3].Width = 150;
                dgvCourse.Columns[4].Width = 150;
                dgvCourse.Columns[5].Width = 250;
                dgvCourse.Columns[6].Width = 50;
            }
        }

        private void DataTable()
        {
            dt = new DataTable();
            dt.Columns.Add("ลำดับ");
            dt.Columns.Add("หมวดวิชา");
            dt.Columns.Add("รหัสวิชา");
            dt.Columns.Add("ชื่อวิชา (อังกฤษ)");
            dt.Columns.Add("ชื่อวิชา (ไทย)");
            dt.Columns.Add("รายละเอียดวิชา");
            dt.Columns.Add("หน่วยกิต");
            
            BindingSource bs = new BindingSource();
            bs.DataSource = dt;
            dgvCourse.DataSource = bs;
        }

        private void ClearData()
        {
            txtName.Text = "";
            txtDetail.Text = "";
            txtID.Text = "";
            txtEng.Text = "";
            txtCredit.Text = "";
            cboType.Text = "";
        }

        private void Clear()
        {
            tsAdd.Enabled = true;
            cboType.Enabled = false;
            txtID.Enabled = false;
            txtName.Enabled = false;
            txtEng.Enabled = false;
            txtDetail.Enabled = false;
            txtCredit.Enabled = false;
            tsOK.Enabled = false;
            tsCancel.Enabled = true;
        }

        private void tsAdd_Click(object sender, EventArgs e)
        {
            ClearData();
            DataTable();
            FormatDgvCourse();

            tsAdd.Enabled = true;
            cboType.Enabled = true;
            txtID.Enabled = true;
            txtName.Enabled = true;
            txtEng.Enabled = true;
            txtDetail.Enabled = true;
            txtCredit.Enabled = true;
            tsOK.Enabled = true;
            tsCancel.Enabled = true;

            txtID.Focus();
        }

        private void tsOK_Click(object sender, EventArgs e)
        {
            if (MessageBox.Show("คุณต้องการบันทึกข้อมูลวิชา ใช่หรือไม่?", "คำยืนยัน", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == System.Windows.Forms.DialogResult.Yes)

            sb = new StringBuilder();
            sb.Append("INSERT INTO Test (CourseName, CourseCode, CourseThai, CourseEng, CourseDetail, Credit) ");
            sb.Append("VALUES (@CourseName, @CourseCode, @CourseThai, @CourseEng, @CourseDetail, @Credit) ");
            Add = sb.ToString();

            Cmd = new SqlCommand(Add, Conn);
            Cmd.CommandType = CommandType.Text;
            Cmd.CommandText = Add;
            Cmd.Connection = Conn;

            for (int i = 0; i <= dgvCourse.RowCount - 1; i++)
            {
                if (!dgvCourse.Rows[i].IsNewRow)
                {
                    Cmd.Parameters.AddWithValue("CourseName", dgvCourse.Rows[i].Cells[1].Value);
                    Cmd.Parameters.AddWithValue("CourseCode", dgvCourse.Rows[i].Cells[2].Value);
                    Cmd.Parameters.AddWithValue("CourseThai", dgvCourse.Rows[i].Cells[3].Value);
                    Cmd.Parameters.AddWithValue("CourseEng", dgvCourse.Rows[i].Cells[4].Value);
                    Cmd.Parameters.AddWithValue("CourseDetail", dgvCourse.Rows[i].Cells[5].Value);
                    Cmd.Parameters.AddWithValue("Credit", dgvCourse.Rows[i].Cells[6].Value);
                }
                Cmd.ExecuteNonQuery();
                Cmd.Parameters.Clear();
            }
        }

        private void tsCancel_Click(object sender, EventArgs e)
        {
            ClearData();
            Clear();
            dgvCourse.DataSource = null;
        }

        private void tsHome1_Click(object sender, EventArgs e)
        {
            frmIndex in4 = new frmIndex();
            in4.Show();
            this.Hide();
        }

        private void btnKo_Click(object sender, EventArgs e)
        {
                id++;
                drw = dt.NewRow();
                drw["ลำดับ"] = id.ToString();
                drw["หมวดวิชา"] = cboType.Text;
                drw["รหัสวิชา"] = txtID.Text;
                drw["ชื่อวิชา (อังกฤษ)"] = txtEng.Text;
                drw["ชื่อวิชา (ไทย)"] = txtName.Text;
                drw["รายละเอียดวิชา"] = txtDetail.Text;
                drw["หน่วยกิต"] = txtCredit.Text;
                dt.Rows.Add(drw);

                if ((txtName.Text.Trim() == "") || (txtDetail.Text.Trim() == "") || (txtID.Text.Trim() == "") || (txtEng.Text.Trim() == "") || (txtCredit.Text.Trim() == ""))
                {
                    MessageBox.Show("กรุณาป้อนข้อมูลให้ครบทุกช่อง !!!", "ผลการตรวจสอบ", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                    txtID.Focus();
                    return;
                }
                if (cboType.Text.Trim() == "")
                {
                    MessageBox.Show("กรุณาเลือกหมวด !!!", "ผลการตรวจสอบ", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
                    cboType.Focus();
                    return;
                }
            }

        public string Add { get; set; }
    }
}