To reproduce:

1. Run the application and click the button. A message box appears. Close the message box.
2. Minimize the form and restore it to normal state.
3. Click the button again. You will encounter an error because the CommandBarDropDownList.SelectedItem property is set to null.

Workaround:

 private void radButton1_Click(object sender, EventArgs e)
 {
     this.commandBarDropDownList1.DropDownAnimationEnabled = false;
     this.commandBarDropDownList1.DropDownListElement.ShowPopup();
     this.commandBarDropDownList1.DropDownListElement.ClosePopup(Telerik.WinControls.UI.RadPopupCloseReason.Mouse);

     Product product = this.commandBarDropDownList1.SelectedItem.DataBoundItem as Product;

     MessageBox.Show(product.Description);
 }