Notice: Undefined index: #fields in Drupal\photoswipe\Plugin\Field\FieldFormatter\PhotoswipeFieldFormatter->settingsForm() in field display settings

Problem:

Notice: Undefined index: #fields in Drupal\photoswipe\Plugin\Field\FieldFormatter\PhotoswipeFieldFormatter->settingsForm()

$form['#fields'] doesn't always have values in the field display form (e.g. admin/structure/types/manage/XXX/display for nodes)

Solution:

Add a !empty() check around $form['#fields']

    // Add the other node fields as options
    if(!empty($form['#fields'])){
      foreach ($form['#fields'] as $node_field) {
        if ($node_field != $this->fieldDefinition->getName()) {
          $caption_options[$node_field] = $node_field;
        }
      }
    }