Not working

It says the magnific library is installed in the status report. And the files are placed correctly.

It looks like the magnific javascript is not initialized at all.

And Eric115, I consider it critical when the module is not working at all. :)

It says the magnific library is installed in the status report. And the files are placed correctly.

It looks like the magnific javascript is not initialized at all.

I consider it "critical" when the module is not working at all and not giving any errors.

I have reinstalled the module and the whole library by deleting folders and reinstalling again - still not initializing.

I use bootstrap theme.

Alright here I have fixed your module which is not working at all at any themes or installations of drupal 8.

Here is the fix:
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1. Add two lines into viewElements function in the file:
src/Plugin/Field/FieldFormatter/MagnificPopup.php

  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    $thumb_image_style = $this->getSetting('thumbnail_image_style');
    $popup_image_style = $this->getSetting('popup_image_style');
    $gallery_type = $this->getSetting('gallery_type');
    $files = $this->getEntitiesToView($items, $langcode);
    foreach ($files as $delta => $file) {
      $image_uri = $file->getFileUri();
      $popup_image_path = !empty($popup_image_style) ? ImageStyle::load($popup_image_style)->buildUrl($image_uri) : $image_uri;
      // Depending on the outcome of https://www.drupal.org/node/2622586,
      // Either a class will need to be added to the $url object,
      // Or a custom theme function might be needed to do so.
      // For the time being, 'a' is used as the delegate in magnific-popup.js.
      $url = Url::fromUri(file_create_url($popup_image_path));
      $item = $file->_referringItem;
      $item_attributes = $file->_attributes;
      unset($file->_attributes);
      $item_attributes['class'][] = 'mfp-thumbnail';
      // ADDED THEESE TWO LINES
      $item_attributes['class'][] = 'mfp-field';      
      $item_attributes['class'][] = 'mfp-' . Html::cleanCssIdentifie;

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js/magnific-popup.js is the following:

(function($, Drupal, drupalSettings) {
  "use strict";
  Drupal.behaviors.magnific_popup = {
    attach: function(context, settings) {
      // Gallery.
      $(context).find('.mfp-all-items, .mfp-first-item, .mfp-random-item').once('mfp-processed').each(function() {
var elem = $(this).parent().parent();
        elem.magnificPopup({
          delegate: 'a',
          type: 'image',
          gallery: {
            enabled: true
          },
          image: {
            titleSrc: 'alt'
          }
        });
      });
      // Separate items.
      $(context).find('.mfp-separate-items').once('mfp-processed').each(function() {
var elem = $(this).parent().parent();
        elem.magnificPopup({
          delegate: 'a',
          type: 'image',
          image: {
            titleSrc: 'alt'
          }
        });
      });
    }
  };
})(jQuery, Drupal, drupalSettings);

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This is not a dublicate, because the library is installed correctly and I have tried your patch and it still doesn't work