WP menu routes undefined

I am following the Frontity Talks 2021-01 - Fetching menus from WordPress, but when my menu is loaded the links are undefined and content of the pages is not loaded. And after one click the links are dead.
Any thoughts?

Hi @s74, can you please show your code to fetch the menu, and also how you are using it?

Hello! Sorry, but it is on localhost so I can’t show it.

But it is the same code as in the tutorial, but without the mars-theme, just a blank theme.

When I did the same solution with mars-theme, it works straight away. So I guess there is something in the theme which is necessary for this tutorial/code to work.