private void inventorylogBttn_Click(object sender, EventArgs e)
 {
     if (inventorylogform == null)
     {
         inventorylogform             = new inventorylogFrm();
         inventorylogform.FormClosed += inventorylogform_FormClosed;
     }
     inventorylogform.Show(this);
     Hide();
 }
 void inventorylogform_FormClosed(object sender, FormClosedEventArgs e)
 {
     inventorylogform = null;
     Show();
 }