private void btnUpdate_Click(object sender, EventArgs e)
        {
            frm_MaintainProduct maintain = new frm_MaintainProduct(GetSelectedProduct(), false);

            maintain.ShowDialog();
            loadData();
        }
        private void btnAdd_Click(object sender, EventArgs e)
        {
            frm_MaintainProduct maintain = new frm_MaintainProduct(null, false);

            maintain.ShowDialog();
            loadData();
        }
        private void btnViewDetail_Click(object sender, EventArgs e)
        {
            frm_MaintainProduct maintain = new frm_MaintainProduct(GetSelectedProduct(), true);

            maintain.ShowDialog();
        }