At a party's request, the court must order witnesses excluded so that they cannot hear other witnesses' testimony. Or the court may do so on its own. But this rule does not authorize excluding:
(a) a party who is a natural person;
(b) an officer or employee of a party that is not a natural person, after being designated as the party's representative by its attorney; or
(c) a person whose presence a party shows to be essential to presenting the party's claim or defense.