private void inventoryBttn_Click(object sender, EventArgs e)
 {
     if (inventoryform == null)
     {
         inventoryform             = new inventoryFrm();
         inventoryform.FormClosed += inventoryform_FormClosed;
     }
     inventoryform.Show(this);
     Hide();
 }
 void inventoryform_FormClosed(object sender, FormClosedEventArgs e)
 {
     inventoryform = null;
     Show();
 }