public FORM_RETURN_MANAGEMENT()
        {
            InitializeComponent();

            if (frm == null) /*بعين قيمة لل form لو قيمتة ب null وهى تكون قيمتها الحالية  */            // .......[4]
            {
                frm = this;
            }

            DGV_RETURN_MANGE.DataSource = return_order.GET_RETURN_ORDER();

            btn_delete.Enabled         = false;
            btn_edit.Enabled           = false;
            btn_return_product.Enabled = false;
        }
        private static FORM_RETURN_MANAGEMENT frm;                                       //  ......................[1]

        static void frm_formclosed(object sender, FormClosedEventArgs e)                 //  ................[2]
        {
            frm = null;
        }
        private void returnBillManagementToolStripMenuItem_Click(object sender, EventArgs e)
        {
            FORM_RETURN_MANAGEMENT frm = new FORM_RETURN_MANAGEMENT();

            frm.ShowDialog();
        }