Fix PHP notices.

bibcite_footnotes.module

@@ -76,7 +76,7 @@ function bibcite_footnotes_preprocess_bibcite_footnote_list(&$variables) {
 
   $notes['#attached']['library'][] = 'bibcite_footnotes/reference_footnote';
   $dont_show_backlink_text = $config['reference_dont_show_backlink_text'];
-  $sort_references_by = $config['reference_sort_by'];
+
   $citation_tools = new CitationTools();
   if (empty($footnotes)) {
     return;
@@ -241,7 +241,7 @@ function bibcite_footnotes_form_node_form_alter(&$form, FormStateInterface &$for
       /**
        * @var FieldConfig $field_config
        */
-      $field_config = $ief_instance['instance'];
+      if ($field_config = $ief_instance['instance']) {
 
         // Check if this is a bibcite_reference field type.
         if ($field_config->getSetting('handler') == 'default:bibcite_reference') {
@@ -261,6 +261,7 @@ function bibcite_footnotes_form_node_form_alter(&$form, FormStateInterface &$for
           }
         }
       }
+    }
 
     $triggering_element = $form_state->getTriggeringElement();
     if (!$triggering_element) {