Strict Standards Error bootstrap navwalker

I am having trouble understanding this error and I’ve not come across it before, please could someone help?

Strict Standards: Declaration of wp_bootstrap_navwalker::start_lvl() should be compatible with Walker_Nav_Menu::start_lvl(&$output, $depth = 0, $args = Array) in .../wp-content/themes/flat-portfolio/wp_bootstrap_navwalker.php on line 143

The wp_bootstrap_navwalker.php code is as follows:

<?php

/**
 * Class Name: wp_bootstrap_navwalker
 * GitHub URI: https://github.com/twittem/wp-bootstrap-navwalker
 * Description: A custom WordPress nav walker class to implement the Twitter Bootstrap 2.3.2 navigation style in a custom theme using the WordPress built in menu manager.
 * Version: 1.4.3
 * Author: Edward McIntyre - @twittem
 * License: GPL-2.0+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 */

class wp_bootstrap_navwalker extends Walker_Nav_Menu {

    /**
     * @see Walker::start_lvl()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param int $depth Depth of page. Used for padding.
     */
    function start_lvl( &$output, $depth ) {
        $indent = str_repeat( "t", $depth );
        $output    .= "n$indent<ul class="dropdown-menu">n";        
    }

    /**
     * @see Walker::start_el()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param object $item Menu item data object.
     * @param int $depth Depth of menu item. Used for padding.
     * @param int $current_page Menu item ID.
     * @param object $args
     */

    function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        global $wp_query;
        $indent = ( $depth ) ? str_repeat( "t", $depth ) : '';

        /**
         * Dividers & Headers
         * ==================
         * Determine whether the item is a Divider, Header, or regular menu item.
         * To prevent errors we use the strcasecmp() function to so a comparison
         * that is not case sensitive. The strcasecmp() function returns a 0 if 
         * the strings are equal.
         */
        if (strcasecmp($item->title, 'divider') == 0) {
            // Item is a Divider
            $output .= $indent . '<li class="divider">';
        } else if (strcasecmp($item->title, 'divider-vertical') == 0) {
            // Item is a Vertical Divider
            $output .= $indent . '<li class="divider-vertical">';
        } else if (strcasecmp($item->title, 'nav-header') == 0) {
            // Item is a Header
            $output .= $indent . '<li class="nav-header">' . esc_attr( $item->attr_title );
        } else {

            $class_names = $value = '';
            $classes = empty( $item->classes ) ? array() : (array) $item->classes;
            $classes[] = ($item->current) ? 'active' : '';
            $classes[] = 'menu-item-' . $item->ID;
            $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );

            if ($args->has_children && $depth > 0) {
                $class_names .= ' dropdown-submenu';
            } else if($args->has_children && $depth === 0) {
                $class_names .= ' dropdown';
            }

            $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

            $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
            $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

            $output .= $indent . '<li' . $id . $value . $class_names .'>';

            $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        ) .'"' : '';
            $attributes .= ($args->has_children)        ? ' data-toggle="dropdown" data-target="#" class="dropdown-toggle"' : '';

            $item_output = $args->before;

            /**
             * Glyphicons
             * ===========
             * Since the the menu item is NOT a Divider or Header we check the see
             * if there is a value in the attr_title property. If the attr_title
             * property is NOT null we apply it as the class name for the glyphicon.
             */
            if(! empty( $item->attr_title )){
                $item_output .= '<a'. $attributes .'><i class="' . esc_attr( $item->attr_title ) . '"></i>&nbsp;';
            } else {
                $item_output .= '<a'. $attributes .'>';
            }

            $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
            $item_output .= ($args->has_children && $depth == 0) ? ' <span class="caret"></span></a>' : '</a>';
            $item_output .= $args->after;

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

    /**
     * Traverse elements to create list from elements.
     *
     * Display one element if the element doesn't have any children otherwise,
     * display the element and its children. Will only traverse up to the max
     * depth and no ignore elements under that depth. 
     *
     * This method shouldn't be called directly, use the walk() method instead.
     *
     * @see Walker::start_el()
     * @since 2.5.0
     *
     * @param object $element Data object
     * @param array $children_elements List of elements to continue traversing.
     * @param int $max_depth Max depth to traverse.
     * @param int $depth Depth of current element.
     * @param array $args
     * @param string $output Passed by reference. Used to append additional content.
     * @return null Null on failure with no changes to parameters.
     */

    function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) {
        if ( !$element ) {
            return;
        }

        $id_field = $this->db_fields['id'];

        //display this element
        if ( is_object( $args[0] ) ) {
           $args[0]->has_children = ! empty( $children_elements[$element->$id_field] );
        }

        parent::display_element($element, $children_elements, $max_depth, $depth, $args, $output);
    }
}

?>

I’m calling the walker by putting the following code in my header.php

<?php 
    wp_nav_menu( array(
        'menu'       => 'top_menu',
        'depth'      => 2,
        'container'  => false,
        'menu_class' => 'nav',
        'fallback_cb' => 'wp_page_menu',
        //Process nav menu using our custom nav walker
        'walker' => new wp_bootstrap_navwalker())
    );
?>

and registering the function in functions.php:

// Register Custom Navigation Walker
require_once('wp_bootstrap_navwalker.php');