NullReferenceException: Object reference not set to an instance of an object

Hi,

I keep receiving this exception when i run the following code. It says the error is coming on line 30. The code still runs perfectly but the error messages are very annoying. Please help!

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class WayCont : MonoBehaviour
{
public List<Transform> waypoints = new List<Transform>();
private Transform targetWaypoint;
private int targetWaypointIndex = 0;
private float minDistance = 1.0f;
private int lastWaypointIndex;

private float movementSpeed = 3.0f;
private float rotationSpeed = 2.0f;
private float distance;

// Start is called before the first frame update
void Start()
{
lastWaypointIndex = waypoints.Count - 1;
targetWaypoint = waypoints[targetWaypointIndex];
}

// Update is called once per frame
void Update()
{
float movementStep = movementSpeed * Time.deltaTime;
float rotationStep = rotationSpeed * Time.deltaTime;

Vector3 directionToTarget = targetWaypoint.position - transform.position;
Quaternion rotationToTarget = Quaternion.LookRotation(directionToTarget);

transform.rotation = Quaternion.Slerp(transform.rotation, rotationToTarget, rotationStep);

float distance = Vector3.Distance(transform.position, targetWaypoint.position);
checkDistance(distance);

transform.position = Vector3.MoveTowards(transform.position, targetWaypoint.position, movementStep);



}

void checkDistance(float currentDistance)
{
if(currentDistance <= minDistance)
{
targetWaypointIndex++;
updateTarget();
}
}

void updateTarget()
{
if (targetWaypointIndex > lastWaypointIndex)
{
targetWaypointIndex = 0;
}
targetWaypoint = waypoints[targetWaypointIndex];
}
}