private void inventoryForm_Closed(object sender, EventArgs e)
 {
     inventoryForm = null;
 }
 private void InitializeInventoryForm()
 {
     if (inventoryForm == null)
     {
         inventoryForm = new Inventory();
     }
     inventoryForm.FormClosed += inventoryForm_Closed;
     inventoryForm.initializeInventoryForm();
 }