why i got NullReferenceException object reference not set to an instance of an object c#

Questions & Answers
1

i new to this c#
i try to make an object to move but instate i got null reference expception

this is my script

using UnityEngine;
using System.Collections;

public class motor : MonoBehaviour
{
public float moveSpeed = 5.0f;
public float drag = 0.5f;
public float terminalRotationSpeed = 25.0f;
public virtualjoystick moveJoystick;

private Rigidbody controller;
private Transform camTransform;

private void Start()
{
    controller = GetComponent<Rigidbody>();
    controller.maxAngularVelocity = terminalRotationSpeed;
    controller.drag = drag;

    camTransform = Camera.main.transform;
}

private void Update()
{
    Vector3 dir = Vector3.zero;

    dir.x = Input.GetAxis("Horizontal");
    dir.z = Input.GetAxis("Vertical");

    if (dir.magnitude > 1)
        dir.Normalize();

    if (moveJoystick.InputDirection != Vector3.zero)
    {
        dir = moveJoystick.InputDirection;
    }

    // Rotate our direction vector with camera
    Vector3 rotatedDir = camTransform.TransformDirection(dir);
    rotatedDir = new Vector3(rotatedDir.x, 0, rotatedDir.z);
    rotatedDir = rotatedDir.normalized * dir.magnitude;

    controller.AddForce(rotatedDir * moveSpeed);
}

}

i got that error in line 39 that is the = Vector3 rotatedDir = camTransform.TransformDirection(dir);

2

because the camTransform is null. It is never set to anything.
Try

[SerializeField] Transform camTransform

then in the inspector set the camera variable to the main camera