NullReferenceException: Object reference not set to an instance of an object. Пожалуйста, помогите исправить ошибку

Понимаю суть ошибки, но не могу исправить.

На строке: controller.Move(horizontalMove * Time.fixedDeltaTime, crouch, jump); возникает ошибка:

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

Сам класс:

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

public class PlayerMovement : MonoBehaviour
{
    public CharacterController2D controller;
    public float runSpeed= 40f;

    bool jump = false;
    bool crouch = false;

    float horizontalMove = 0f;

    void Update()
    {
        horizontalMove = Input.GetAxisRaw("Horizontal") * runSpeed;
        if (Input.GetButtonDown("Jump"))   
        {
            jump = true;
        }

        if (Input.GetButtonDown("Crouch"))
        {
            crouch = true;
        }
        if (Input.GetButtonUp("Crouch"))
        {
            crouch = false;
        }
    }
    void FixedUpdate()
    {
        //Move our character
        controller.Move(horizontalMove * Time.fixedDeltaTime, crouch, jump); //тут ошибка.
        jump = false;
        crouch = false;
    }
}