Replacing the NavWalker dropdown element

You need to replace the end_lvl method of Walker_Nav_Menu as well. This is what you would need to add:

public function end_lvl( &$output, $depth = 0, $args = array() ) {
    if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
        $t="";
        $n = '';
    } else {
        $t = "\t";
        $n = "\n";
    }
    $indent = str_repeat( $t, $depth );
    $output .= "$indent</div>{$n}";
}

The only change from the original is the </ul> was changed to </div>