Replace the code in the plugin with this code 

using UnityEngine;

namespace Oxide.Plugins {
[Info("Extra Seating", "Pho3niX90", "1.1.2")]
[Description("Allows extra seats on minicopters, attackcopters and horses")]
class ExtraSeating : RustPlugin {
#region Config
public PluginConfig config;
static ExtraSeating _instance;
bool debug = false;
int seats = 0;

protected override void LoadDefaultConfig() { Config.WriteObject(GetDefaultConfig(), true); }
public PluginConfig GetDefaultConfig() { return new PluginConfig { EnableMiniSideSeats = true, EnableMiniBackSeat = true, EnableExtraHorseSeat = true, EnableAttackSideSeats = true }; }
public class PluginConfig { public bool EnableMiniSideSeats; public bool EnableMiniBackSeat; public bool EnableExtraHorseSeat; public bool EnableAttackSideSeats; }
#endregion
private void Init() {
config = Config.ReadObject<PluginConfig>();
}

void LogDebug(string str) {
if (debug) Puts(str);
}

void OnEntitySpawned(BaseNetworkable entity) {
_instance = this;
if (entity == null || !(entity is Minicopter || entity is RidableHorse || entity is AttackHelicopter)) return;
BaseVehicle vehicle = entity as BaseVehicle;
seats = vehicle.mountPoints.Count; // default

if (entity is Minicopter && entity.ShortPrefabName.Equals("minicopter.entity"))
{

if (_instance.config.EnableMiniSideSeats) seats += 2;
if (_instance.config.EnableMiniBackSeat) seats += 1;

if (vehicle.mountPoints.Count < seats)
vehicle?.gameObject.AddComponent<Seating>();
}

if (entity is AttackHelicopter && entity.ShortPrefabName.Equals("attackhelicopter.entity"))
{

if (_instance.config.EnableAttackSideSeats) seats += 2;

if (vehicle.mountPoints.Count < seats)
vehicle?.gameObject.AddComponent<Seating>();
}

if (entity is RidableHorse) {
if (_instance.config.EnableExtraHorseSeat) seats += 1;
if (vehicle.mountPoints.Count < seats)
vehicle?.gameObject.AddComponent<Seating>();
}
}

void AddSeat(BaseVehicle ent, Vector3 locPos, Quaternion q) {
BaseEntity seat = GameManager.server.CreateEntity("assets/prefabs/vehicle/seats/passengerchair.prefab", ent.transform.position, q) as BaseEntity;
if (seat == null) return;

seat.SetParent(ent);
seat.Spawn();
seat.transform.localPosition = locPos;
seat.SendNetworkUpdateImmediate();
}

BaseVehicle.MountPointInfo CreateMount(Vector3 vec, BaseVehicle.MountPointInfo exampleSeat, Vector3 rotation) {
return new BaseVehicle.MountPointInfo {
pos = vec,
rot = rotation,
bone = exampleSeat.bone,
prefab = exampleSeat.prefab,
mountable = exampleSeat.mountable
};
}

#region Classes

class Seating : MonoBehaviour {
public BaseVehicle entity;
void Awake() {
entity = GetComponent<BaseVehicle>();
bool isMini = entity is Minicopter;
bool isHorse = entity is RidableHorse;
Vector3 emptyVector = new Vector3(0, 0, 0);
if (isMini) {
_instance.LogDebug("Minicopter detected");
}
if (isHorse) {
_instance.LogDebug("Horse detected");
}

if (entity == null) { Destroy(this); return; }

BaseVehicle.MountPointInfo pilot = entity.mountPoints[0];
//entity.mountPoints.Clear();

if (entity is RidableHorse) {
_instance.LogDebug("Adding passenger seat");
Vector3 horseVector = new Vector3(0f, -0.32f, -0.5f);
BaseVehicle.MountPointInfo horseBack = _instance.CreateMount(horseVector, pilot, emptyVector);
//entity.mountPoints.Add(pilot);
entity.mountPoints.Add(horseBack);
entity.SendNetworkUpdateImmediate();
}

if (entity is Minicopter) {
BaseVehicle.MountPointInfo pFront = entity.mountPoints[1];
Vector3 leftVector = new Vector3(0.6f, 0.2f, -0.2f);
Vector3 rightVector = new Vector3(-0.6f, 0.2f, -0.2f);
Vector3 backVector = new Vector3(0.0f, 0.4f, -1.2f);
Vector3 backVector2 = new Vector3(0.0f, 0.4f, -1.45f);

Vector3 playerOffsetVector = new Vector3(0f, 0f, -0.25f);
Quaternion backQuaternion = Quaternion.Euler(0f, 180f, 0f);

if (_instance.config.EnableMiniSideSeats) {
_instance.LogDebug("Adding side seats");
BaseVehicle.MountPointInfo pLeftSide = _instance.CreateMount(leftVector, pFront, emptyVector);
BaseVehicle.MountPointInfo pRightSide = _instance.CreateMount(rightVector, pFront, emptyVector);
entity.mountPoints.Add(pLeftSide);
entity.mountPoints.Add(pRightSide);
_instance.AddSeat(entity, leftVector + playerOffsetVector, new Quaternion());
_instance.AddSeat(entity, rightVector + playerOffsetVector, new Quaternion());
}

if (_instance.config.EnableMiniBackSeat) {
_instance.LogDebug("Adding back/rotor seat");
BaseVehicle.MountPointInfo pBackReverse = _instance.CreateMount(backVector2, pFront, new Vector3(0f, 180f, 0f));
entity.mountPoints.Add(pBackReverse);
_instance.AddSeat(entity, backVector, backQuaternion);
}
}

if(entity is AttackHelicopter)
{
BaseVehicle.MountPointInfo pFront = entity.mountPoints[1];
Vector3 leftVector = new Vector3(1.1f, 0.7f, 0.3f);
Vector3 rightVector = new Vector3(-1.1f, 0.7f, 0.3f);

Vector3 playerOffsetVector = new Vector3(0f, 0f, -0.25f);

_instance.LogDebug("Adding side seats");
BaseVehicle.MountPointInfo pLeftSide = _instance.CreateMount(leftVector, pFront, emptyVector);
BaseVehicle.MountPointInfo pRightSide = _instance.CreateMount(rightVector, pFront, emptyVector);
entity.mountPoints.Add(pLeftSide);
entity.mountPoints.Add(pRightSide);
_instance.AddSeat(entity, leftVector + playerOffsetVector, new Quaternion());
_instance.AddSeat(entity, rightVector + playerOffsetVector, new Quaternion());
}

}
}
#endregion
}
}