The plugin has generated 2 unexpected output characters during activation

Question

I'm trying to make my first widget/plugin, the plugin works well, but, after the activation it sends me this error:

"The plugin has generated 2 unexpected output characters during activation. If you notice “headers already sent” messages, issues with syndication feeds, or other problems, try disabling or removing this plugin."

Then, when I enter to the site it sends me this other error:

"Content encoding error

The page you are trying to view cannot be displayed because it uses an invalid or unsupported compression format."

After that, if I refresh the page, it let my enter normally, but, the same issue is repeated every time I enter to a different page.

I don't understand what's happen, so I will appreciate your help to solve this.

This is the code:

<?php


/*
Plugin Name: Mx Energy Partners - Post Relacionados
Plugin URI:
Description: Añade Post Relacionados al sitio Mx Energy Partners
Version: 1.0.0
Author: Arturo Valverde
Author URI:
Text Domain: mxtechnologypartners 
*/

?>

<?php
if (!defined('ABSPATH')) die();

class Mxnrgprtnrs_related_posts extends WP_Widget
{

  public function __construct()
  {
    $widget_ops = array(
      'classname' => 'mxnrgprtnrs_posts_relacionados',
      'description' => 'Añade Post Relacionados al sitio Mx Energy Partners',
    );
    parent::__construct(
      'mxnrgprtnrs_posts_relacionados',
      'MX Energy Partners - Posts Relacionados',
      $widget_ops
    );
  }



  public function widget($args, $instance)
  {

    if (post_type_exists('post')) : ?>      

      <div class="related-posts blog padding-5">

        <h4>Artículos Relacionados:</h4>

        <ul class="listado-blog">
          <?php

          $tags = get_the_tags();

          if ($tags) :
            $tag_ids = array();

            foreach ($tags as $tag) {
              $tag_ids[] = $tag->term_id;
            }

            $qryargs = array(
              'post_type' => 'post',
              'tag__in' => $tag_ids,
              'post__not_in' => array(get_the_ID()),
              'orderby' => 'post_date',
              'order' => 'DESC',
              'posts_per_page' => 4,
              'post_status' => 'publish',
            );

            $my_query = new WP_Query($qryargs);

            if ($my_query->have_posts()) : while ($my_query->have_posts()) : $my_query->the_post();
          ?>

                <li class="blog-item">

                  <div class="img-post">
                    <a href="<?php the_permalink(); ?>">
                      <?php the_post_thumbnail('thumbnail'); ?>
                    </a>
                  </div>

                  <div class="contenido">

                    <div class="meta full-width">
                      <a href="<?php the_permalink(); ?>">

                        <h4><?php the_title(); ?></h4>

                      </a>

                      <span>
                        Autor:

                        <a href="<?php echo get_author_posts_url(get_the_author_meta('ID')) ?>">
                          <?php echo get_the_author_meta('display_name'); ?>
                        </a>
                      </span>

                      <span>
                        <?php the_time('l d, F, Y'); ?>
                      </span>


                    </div>


                  </div>

                </li>


          <?php

              endwhile;
            endif;

          else :
            wp_reset_query();

          endif;



          wp_reset_query();
          ?>
        </ul>
      </div>

    <?php endif; ?>

<?php
  }


  public function form($instance)
  {
    // outputs the options form on admin
  }

  public function update($new_instance, $old_instance)
  {
    // processes widget options to be saved
  }
}

add_action('widgets_init', function () {
  register_widget('mxnrgprtnrs_related_posts');
});

?>

Answer 1

Remove the closing and re-opening PHP tags between the plugin information and the class declaration.:

?>

<?php

That code is essentially emitted white space which is where your extra characters are coming from.