Hi, I resolve bug removing the & from &$output in the two lines:
parent::start_el( &$output, $item, $depth, $args );
Anyway, I change the code because I have conflict with my theme. The theme add ‘wp_nav_menu_args’ filter too.
Then I lose all menu format.
What I do is remove the ‘wp_nav_menu_args’ filter and register to ‘walker_nav_menu_start_el’.
And all work good.