Wordpress Widget : List pages of parent page

functions.php

<?php 

function template_register_widget() {
	register_widget( 'my_widget' );
}

add_action( 'widgets_init', 'template_register_widget' );

class my_widget extends WP_Widget {
	function __construct() {
		parent::__construct(
			// widget ID
			'my_widget',
			// widget name
			__('List pages of parent page', ' my_widget_domain'),
			// widget description
			array( 'description' => __( 'List pages of same level', 'my_widget_domain' ), )
		);
	}
	public function widget( $args, $instance ) {
		$title = apply_filters( 'widget_title', $instance['title'] );
		echo $args['before_widget'];
		//if title is present
		if ( ! empty( $title ) )
			echo $args['before_title'] . $title . $args['after_title'];
		//output
		global $post;
		$parentPage = $post->post_parent;
		$currentSlug = $post->post_name;
		$args = array(
			'post_type'      => 'page',
			'posts_per_page' => -1,
			'post_parent'    => $parentPage,
			'order'          => 'ASC',
			'orderby'        => 'menu_order'
		);
		$parent = new WP_Query( $args );
		if ( $parent->have_posts() ) : ?>

			<?php while ( $parent->have_posts() ) : $parent->the_post(); ?>

			<?php $postSlug = $post->post_name;
			if($currentSlug == $postSlug){
				echo'<div id="parent-'. get_the_ID() .'" class="level-page current-menu">';
			}
			else{
				echo'<div id="parent-'. get_the_ID() .'" class="level-page">';
			} ?>
				<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a>

				</div>

			<?php endwhile; ?>

		<?php endif; wp_reset_postdata();
		//echo __( 'Hello, World from Hostinger.com', 'my_widget_domain' );
		echo $args['after_widget'];
	}
	public function form( $instance ) {
		if ( isset( $instance[ 'title' ] ) )
			$title = $instance[ 'title' ];
		else
			$title = __( 'Default Title', 'my_widget_domain' );
		?>
		<p>
			<label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label>
			<input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>" />
		</p>
		<?php
	}
	public function update( $new_instance, $old_instance ) {
		$instance = array();
		$instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';
		return $instance;
	}
}