private void btnThem_Click(object sender, EventArgs e)
        {
            ThaoTac = "themmoi";
            GUI_ThemThanhVien themTV = new GUI_ThemThanhVien();

            themTV.ShowDialog();
        }
 private void btnSua_Click(object sender, EventArgs e)
 {
     if (_maThanhVien <= 0)
     {
         MessageBox.Show("Vui lòng chọn thành viên cần sửa!");
     }
     else
     {
         ThaoTac = "sua";
         GUI_ThemThanhVien SuaTV = new GUI_ThemThanhVien();
         SuaTV.ShowDialog();
     }
 }