WordPress custom nav walker check if item has sub menu

I am using a custom nav walker and would like to create a tree menu.

Example:

<ul class="nav nav-list">
    <li><label class="tree-toggler nav-header">Header 1</label>
        <ul class="nav nav-list tree">
            <li><a href="#">Link</a></li>
            <li><a href="#">Link</a></li>
            <li><label class="tree-toggler nav-header">Header 1.1</label>
                <ul class="nav nav-list tree">
                    <li><a href="#">Link</a></li>
                    <li><a href="#">Link</a></li>
                    <li><label class="tree-toggler nav-header">Header 1.1.1</label>
                        <ul class="nav nav-list tree">
                            <li><a href="#">Link</a></li>
                            <li><a href="#">Link</a></li>
                        </ul>
                    </li>
                </ul>
            </li>
        </ul>
    </li>
</ul>

Now my goal is to check if a list item has a sub menu and if it has a sub menu I want to wrap the li’s text in a label to create the collapse effect.

Here’s my custom nav walker

class themeslug_walker_nav_menu extends Walker_Nav_Menu {

// add classes to ul sub-menus
function start_lvl( &$output, $depth ) {
    // depth dependent classes
    $indent = ( $depth > 0  ? str_repeat( "t", $depth ) : '' ); // code indent
    $display_depth = ( $depth + 1); // because it counts the first submenu as 0
    $classes = array(
        'sub-menu',
        ( $display_depth % 2  ? 'menu-odd' : 'menu-even' ),
        ( $display_depth >=2 ? 'sub-sub-menu' : '' ),
        'menu-depth-' . $display_depth
        );

    $class_names = implode( ' ', $classes );

    // build html
    $output .= "n" . $indent . '<ul class="' . $class_names . '">' . "n";
}

// add main/sub classes to li's and links
 function start_el( &$output, $item, $depth, $args ) {
    global $wp_query;
    $indent = ( $depth > 0 ? str_repeat( "t", $depth ) : '' ); // code indent

    // depth dependent classes
    $depth_classes = array(
        ( $depth == 0 ? 'topElement' : 'parent' ),
        ( $depth >=2 ? 'sub-sub-menu-item' : '' ),
        ( $depth % 2 ? 'menu-item-odd' : 'menu-item-even' ),
        'menu-item-depth-' . $depth
    );
    $depth_class_names = esc_attr( implode( ' ', $depth_classes ) );

    // passed classes
    $classes = empty( $item->classes ) ? array() : (array) $item->classes;
    $class_names = esc_attr( implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) ) );


    if($depth === 0){
         // build html
        $output .= $indent . '<li id="nav-menu-item-'. $item->ID . '" class="' . $depth_class_names . ' ' . $class_names . '"><label class="tree-toggler">'.$item->title.'</label>';
    }else{
         $output .= $indent . '<li id="nav-menu-item-'. $item->ID . '" class="' . $depth_class_names . ' ' . $class_names . '">';
    };


    // link attributes
    $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
    $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
    $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
    $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
    $attributes .= ' class="menu-link ' . ( $depth > 0 ? 'sub-menu-link' : 'main-menu-link' ) . '"';

    $item_output = sprintf( '%1$s<a%2$s>%3$s%4$s%5$s</a>%6$s',
        $args->before,
        $attributes,
        $args->link_before,
        apply_filters( 'the_title', $item->title, $item->ID ),
        $args->link_after,
        $args->after
    );

    // build html
    $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
}
}

This works fine but only for the li’s in my first depth.
So I am looking for a check if the li has a sub menu or not

Is there anyone who can help me out please.

Thanks in advance!