Print order needs render_with_wicked_pdf

and call WickedPdf::PdfHelper explicitly

app/controllers/spree/admin/orders_controller.rb

@@ -89,7 +89,7 @@ module Spree
       end
 
       def print
-        render InvoiceRenderer.new.args(@order)
+        render_with_wicked_pdf InvoiceRenderer.new.args(@order)
       end
 
       def print_ticket