drupal_get_messages may returns MarkupInterface objects as messages in D8

Just tested Google Analytics migration from D7 to D8 and after the migration completed I got this message. I need to say that this is not my fault in GA. I have only seen this after the Drupal migration completed and no where else.

Upgrade process not completed
Review the detailed upgrade log
Warning: strip_tags() expects parameter 1 to be string, object given in google_analytics_page_attachments() (line 150 of modules\google_analytics\google_analytics.module).

The error comes from this code Json::encode(strip_tags($message)):

      foreach (drupal_get_messages(NULL, FALSE) as $type => $messages) {
        // Track only the selected message types.
        if (in_array($type, $message_types)) {
          foreach ($messages as $message) {
            // @todo: Track as exceptions?
            $message_events .= 'ga("send", "event", ' . Json::encode(t('Messages')) . ', ' . Json::encode($status_heading[$type]) . ', ' . Json::encode(strip_tags($message)) . ');';
          }
        }
      }

A message need to be TEXT only and cannot be a OBJECT. I guess the migration UI or Core has a bug.