Show button for creating invoices also at finished orders

Check if a order is not opened instead of checking if it is closed.

app/views/orders/show.html.haml

@@ -4,7 +4,7 @@
   - content_for :actionbar do
     - if @order.invoice.present?
       = link_to t('.show_invoice'), finance_invoice_path(@order.invoice), class: 'btn'
-    - elsif @order.closed?
+    - elsif !@order.open?
       = link_to t('.create_invoice'), new_finance_invoice_path(:order_id => @order, :supplier_id => @order.supplier),
         class: 'btn'