Walker problem : Illegal offset …

Hi guys !

I’m actually trying to create my own theme. For this, I use the framework Materialize. There isn’t the problem.

To have a nice sidebar, I must get the output of wp_nav_menu() like this :

<ul class='collapsible collapsbile-accordion'>
	<li class='bold'>
		<a class='collapsible-header waves-effect'>$object->title</a>
		<div class='collapsible-body'>
			<ul>
				<li>
					<a href='$object->url'>$object->title</a>
				</li>
			</ul>
		</div>
	</li>
</ul>

So, I tried to create a Walker that would make this work.

class Sidebar_Walker extends Walker {

        function start_lvl( &$output, $depth, $args ) {

            if( $depth == 0 )
                $output .= "<ul class='collapsible collapsbile-accordion'>";
            else
                $output .= "<div class='collapsible-body'><ul>";
        }

        function end_lvl( &$output, $depth, $args )  {

            if( $depth == 0 )
                $output .= "</ul>";
            else
                $output .= "</ul></div>";
        }

        function start_el( &$output, $item, $depth, $args )  {

            if( $depth == 0 )
                $output .= "<li class='bold'><a class='collapsible-header waves-effect'>".$item->title."</a>";
            else
                $output .= "<li><a href='".$item->url."'>".$item->title."</a>";
        }

        function end_el( &$output, $item, $depth, $args ) {

            $output .= "</li>";
        }
}

I’m not confident in this code to work and, when I set the Walker as callback of wp_nav_menu(), I just get this error :

Warning: Illegal offset type in D:\XAMPP\htdocs\DIVTEC\wp-includes\class-wp-walker.php on line 224.

Line 224 is : $children_elements[ $e->$parent_field ][] = $e;

Thanks for reading this, I hope that you can help me !

(Sorry for english mistakes I could have done, it isn’t my main language)