BuddyPress.org

That’s an old bug who was patched. (see here) Check bbpress code and update it if you’re using an older version.

Also, if you have debug mode on in wp-config, revert it to off.
If the message still remains, deactivate your theme and use Twenty Fifteen or Fourteen to test.
If the message disappear, you know the culprit. In that case , contact your theme support