word “undefined” instead of black triangle in mobile menu

In the menu in the mobile version of my site robertpersson.eu, where there should be black triangles to indicate sub-menus I instead see the word “undefined” when I first pull down the menu. If I then open and close the sub-menu, the black triangle is now correctly rendered and the word “undefined” is gone.

How can I make the menu immediately correctly display the black triangles?

The page I need help with: [log in to see the link]