Issue #3487516 by mably: Warning shown on page when purging caches

src/Plugin/Field/FieldFormatter/AbleplayerRemoteVideoFormatter.php

@@ -103,6 +103,9 @@ class AbleplayerRemoteVideoFormatter extends FormatterBase {
     if ($parent->hasField('ableplayer_audio_description')) {
       $audio_description_url = $parent->ableplayer_audio_description->value;
     }
+    else {
+      $audio_description_url = NULL;
+    }
 
     foreach ($items as $delta => $item) {
       $main_property = $item->getFieldDefinition()->getFieldStorageDefinition()->getMainPropertyName();