private void listCasher_Click(object sender, EventArgs e)
 {
     if (staff_form != null)
     {
         if (staff_form.IsDisposed)
         {
             staff_form = null;
         }
     }
     if (staff_form == null)
     {
         staff_form = new StaffDetailOfHuy();
         staff_form.main_staff_dto = (StaffDTO)listCasher.SelectedItem;
         staff_form.LoadData();
         staff_form.Show();
     }
     else
     {
         staff_form.main_staff_dto = (StaffDTO)listCasher.SelectedItem;
         staff_form.LoadData();
     }
 }
 private void btnShowStaffDetail_Click(object sender, EventArgs e)
 {
     staff_form.Show();
 }