Display data on front-end not working, documentation unclear

The previous thread was not satisfactory and didn’t help me at all.

Here’s my header.php Walker, which is pretty standard:

<?php
        if (has_nav_menu('primary_navigation')) :
          wp_nav_menu([
            'theme_location' => 'primary_navigation',
            'menu_class' => 'nav navbar-nav'
            ]);
        endif;
        ?>

But when I implement emangham‘s solution I not only get multiple property errors, it also fails to display the data from the back-end.

This doesn’t seem like a very reliable solution/plugin.

Can someone show me how to implement/modify my walker to display these Nav Fields data on the front-end? The documentation is not very clear. I can create fields, enter data, save them just fine.. but then what? I just cannot get this data to appear on the front-end at all.