Ticket #39888: 39888.1.diff

wp-admin/includes/post.php

@@ -1462 +1462 @@
         $time = $lock[0];
         $user = isset( $lock[1] ) ? $lock[1] : get_post_meta( $post->ID, '_edit_last', true );
 
+        if ( get_userdata( $user ) === false )
+                return false;
+
         /** This filter is documented in wp-admin/includes/ajax-actions.php */
         $time_window = apply_filters( 'wp_check_post_lock_window', 150 );