private void btn_add_Click(object sender, EventArgs e)
        {
            FormUnitAdd form = new FormUnitAdd();

            form.ShowDialog();

            updateUnitTable();
        }
        private void btn_change_Click(object sender, EventArgs e)
        {
            int  PK_Unit      = Convert.ToInt32(dgv_units.SelectedRows[0].Cells[2].Value);
            Unit changingUnit = Program.db.Units.Find(PK_Unit);

            if (changingUnit != null)
            {
                FormUnitAdd form = new FormUnitAdd(changingUnit);
                form.ShowDialog();

                updateUnitTable();
            }
        }