private void mnuAddSupplier_Click(object sender, EventArgs e)
 {
     if (supForm != null)
     {
         supForm.Show();
     }
     else
     {
         supForm           = new SupplierForm();
         supForm.MdiParent = clsFormProvider.mainF;
         supForm.Show();
         supForm.FormClosing += supForm_FormClosing;
     }
 }
 public void userForm_FormClosing(object sender, FormClosingEventArgs e)
 {
     supForm = null;
 }