private void mnuFormsTables_Click(object sender, EventArgs e)
        {
            bool isOpen = false;

            foreach (Form f in Application.OpenForms)
            {
                if (f.Name == "frmTables")
                {
                    isOpen = true;
                    f.Focus();
                    break;
                }
            }
            if (isOpen == false)
            {
                frmTables aTables = new frmTables();
                aTables.MdiParent = this;
                aTables.Show();
                aTables.Focus();
            }
        }
 private void frmTables_FormClosing(object sender, FormClosingEventArgs e)
 {
     frmTablesInstance = null;
 }