[#3276091] Fixes undefined index.

Closes #3276091

src/Plugin/Filter/FilterWebpurifyProfanity.php

@@ -42,7 +42,7 @@ class FilterWebpurifyProfanity extends FilterBase {
       '#type' => 'select',
       '#options' => webpurify_modes_list(),
       '#description' => $this->t('Select how you want WebPurify to handle fields.'),
-      '#default_value' => $this->settings['webpurify_mode']
+      '#default_value' => $this->settings['webpurify_mode'] ?? WEBPURIFY_REWRITE_MODE,
     ];
 
     $form['webpurify_validation_message'] = [