This is an example plugin for Kinsta blog readers

kinsta-widget.php

<?php
/**
 * @package Kinsta_widget
 * @version 1.0
 */
/*
Plugin Name: Kinsta Widget
Plugin URI: http://wordpress.org/extend/plugins/#
Description: This is an example plugin 
Author: Your Name
Version: 1.0
Author URI: http://example.com/
*/

class Kinsta_Widget extends WP_Widget {

	/**
	 * Sets up the widgets name etc
	 *
	 * @link https://developer.wordpress.org/reference/classes/wp_widget/__construct/
	 * @see https://developer.wordpress.org/reference/functions/wp_register_sidebar_widget/
	 *
	 */
	public function __construct() {
		$widget_ops = array( 
			'classname' => 'kinsta_widget',
			'description' => 'A plugin for Kinsta blog readers',
		);
		parent::__construct( 'kinsta_widget', 'Kinsta Widget', $widget_ops );
	}

	/**
	 * Outputs the content of the widget on front-end
	 *
	 * @param array $args Widget arguments 
	 * @param array $instance 
	 *
	 * @link https://developer.wordpress.org/reference/classes/wp_widget/widget/
	 */
	public function widget( $args, $instance ) {
		echo $args['before_widget'];
		if ( ! empty( $instance['title'] ) ) {
			echo $args['before_title'] . apply_filters( 'widget_title', $instance['title'] ) . $args['after_title'];
		}

		if( ! empty( $instance['selected_posts'] ) && is_array( $instance['selected_posts'] ) ){ 

			$selected_posts = get_posts( array( 'post__in' => $instance['selected_posts'] ) );
			?>
			<ul>
			<?php foreach ( $selected_posts as $post ) { ?>
				<li><a href="<?php echo get_permalink( $post->ID ); ?>"><?php echo $post->post_title; ?></a></li>
			<?php } ?>
			</ul>
			<?php 
		
		}else{
			echo esc_html__( 'No posts selected!', 'text_domain' );	
		}

		echo $args['after_widget'];
	}

	/**
	 * Outputs the options form on admin
	 *
	 * @param array $instance The widget options
	 *
	 * @link https://developer.wordpress.org/reference/classes/wp_widget/form/
	 */
	public function form( $instance ) {
		$title = ! empty( $instance['title'] ) ? $instance['title'] : esc_html__( 'Title', 'text_domain' );
		?>
		<p>
		<label for="<?php echo esc_attr( $this->get_field_id( 'title' ) ); ?>"><?php esc_attr_e( 'Title:', 'text_domain' ); ?></label> 
		<input class="widefat" id="<?php echo esc_attr( $this->get_field_id( 'title' ) ); ?>" name="<?php echo esc_attr( $this->get_field_name( 'title' ) ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>">
		</p>
		<p>
		<?php
		
		$posts = get_posts( array( 
				'posts_per_page' => 20,
				'offset' => 0
			) );

		$selected_posts = ! empty( $instance['selected_posts'] ) ? $instance['selected_posts'] : array();
		?>
		<div style="max-height: 120px; overflow: auto;">
		<ul>
		<?php foreach ( $posts as $post ) { ?>
			<li><input type="checkbox" name="<?php echo esc_attr( $this->get_field_name( 'selected_posts' ) ); ?>[]" value="<?php echo $post->ID; ?>" <?php checked( ( in_array( $post->ID, $selected_posts ) ) ? $post->ID : '', $post->ID ); ?> /><?php echo get_the_title( $post->ID ); ?></li>
		<?php } ?>
		</ul>
		</div>
		<?php
		
	}

	/**
	 * Processing widget options on save
	 *
	 * @param array $new_instance The new options
	 * @param array $old_instance The previous options
	 *
	 * @link https://developer.wordpress.org/reference/classes/wp_widget/update/
	 */
	public function update( $new_instance, $old_instance ) {
		$instance = array();
		$instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';
		
		$selected_posts = ( ! empty ( $new_instance['selected_posts'] ) ) ? (array) $new_instance['selected_posts'] : array();
		$instance['selected_posts'] = array_map( 'sanitize_text_field', $selected_posts );

		return $instance;
	}

}

// register Kinsta_Widget
add_action( 'widgets_init', function(){
	register_widget( 'Kinsta_Widget' );
});