Check for bibliography node in style XML and only add to list if it e…

…xists.

src/Plugin/Field/FieldFormatter/StrawberryCitationFormatter.php

@@ -144,12 +144,16 @@ public function settingsForm(array $form, FormStateInterface $form_state) {
  $citation_style_directory = $this->cslStylesPath;
  // Get the list of style files.
  $style_list = $this->fileSystem->scanDirectory($citation_style_directory, '/\.(csl)$/i', ['recurse' => FALSE, 'key' => 'name']);
- # Generate a list of select options and push in the styles.
+ // Generate a list of select options and push in the styles.
  foreach($style_list as $style) {
  $style_name = $style->name;
  $style_xml = simplexml_load_file($style->uri);
- $style_title = $style_xml->info->title->__toString();
- $style_options[$style_name] = $this->t($style_title);
+ // Check if the bibliography node exists and only add to the list if it does.
+ $style_bibliography_exists = isset($style_xml->bibliography);
+ if($style_bibliography_exists) {
+ $style_title = $style_xml->info->title->__toString();
+ $style_options[$style_name] = $this->t($style_title);
+ }
  }
  // Alphabetize them.
  asort($style_options);