Update ai-global-activity.php

ai-global-activity.php

@@ -78,7 +78,7 @@ function ai_activity_publish_post($post_id) {
   $post_excerpt = get_the_excerpt($post);
   $post_author = apply_filters('the_author', $post->post_author);
   $post_date = get_the_date('Y-m-d H:i:s', $post);
-  $post_thumbnail_url = get_the_post_thumbnail_url($post);
+  $post_thumbnail_url = get_the_post_thumbnail_url($post, 'thumbnail');
   // Rewrite false to null in the get_the_post_thumbnail_url() response.
   if (!$post_thumbnail_url) {
     $post_thumbnail_url = null;