Soil-nav-walker breaks my my footer menu

I am quite a newbie with sage and soil. But was instantly amazed and we just yesterday bought a license to use it our site.
I know have an issue with Soil and my theme with displaying a specific menu.

I use soil 3.6.0 and i have a sage based theme with a custom menu location footer, properly registered in setup.php.

 register_nav_menus([
   'primary_navigation' => __('Primary Navigation', 'sage'),
   'footer' => esc_html__( 'Footer Menu', 'fahrradjaeger' )
  ]);

And then display in my theme’s footer:

<?php wp_nav_menu( array( 'menu' => 'menu-footer', 'menu_class' => 'fj-bottomline-menu','theme_location' => 'footer' ) ); ?>

The menu is nothing special, just a list (no submenus) of custom urls.

When I enable the soil-nav-walker in the theme, instead of the menu i get the following errors:

Notice:  Trying to get property of non-object in /srv/www/wordpress-default/wp-includes/nav-menu-template.php on line 151

Notice:  Trying to get property of non-object in /srv/www/wordpress-default/wp-includes/nav-menu-template.php on line 154

Notice:  Trying to get property of non-object in /srv/www/wordpress-default/wp-includes/nav-menu-template.php on line 154

Notice:  Trying to get property of non-object in /srv/www/wordpress-default/wp-includes/nav-menu-template.php on line 156

Soil and Sage-based theme play nice if i disable the nav-walker thought. Interestingly, the other menus get displayed fine. I just wanted to know: Is it possible i am doing something wrong or should i post an issue over at soil’s github.

You’re seeing this error because you haven’t opened up the admin and set the location for a footer menu yet

Go to http://your-site.com/wp/wp-admin/nav-menus.php?action=locations and set it

That’s the reason why we wrap the primary nav in an if in Sage: