How do you fix NullReferenceException: Object reference not set to an instance of an object... errors?

Hello. I am trying to create a 2d video game where the player can turn right and left using arrow keys, and shoots a harpoon when the space bar is pressed. However, I can't figure out how to make the harpoon shoot into the direction the player is facing because I keep getting this error-

 NullReferenceException: Object reference not set to an instance of an object
 playerMove.FixedUpdate () (at Assets/Scripts/playerMove.cs:62)

What I want to know is

What is NullReferenceExeption?

How do I fix my code to work?

Thanks in advance- George :)

Here is the code I used-

playerMove.cs (script)

 using UnityEngine;
 using System.Collections;
 
 public class playerMove : MonoBehaviour {
 
     // All Variables
     public float speed = 10;
     private Rigidbody2D rigidBody2D;
     private GameObject harpoon_00001;
     private bool facingRight = true;
 
     void Awake () {
 
         rigidBody2D = GetComponent<Rigidbody2D>();
         harpoon_00001 = GameObject.Find("harpoon_00001");
 
     }
 
 void Update () {
 
     if (Input.GetKeyDown(KeyCode.LeftArrow) && !facingRight) {
         Flip();
     }
 
     if (Input.GetKeyDown(KeyCode.RightArrow) && facingRight) {
         Flip();
     }
 
 }
 
 void Flip () {
 
     facingRight = !facingRight;
 
     Vector3 theScale = transform.localScale;
     theScale.x *= -1;
     transform.localScale = theScale;
 
 }
 
     void FixedUpdate () {
         float xMove = Input.GetAxis("Horizontal");
         float yMove = Input.GetAxis("Vertical");
 
         float xSpeed = xMove * speed;
         float ySpeed = yMove * speed;
 
         Vector2 newVelocity = new Vector2(xSpeed, ySpeed);
 
         rigidBody2D.velocity = newVelocity;
 
         if (Input.GetKeyDown("space")) {
             GetComponent<AudioSource>().Play();
             
 harpoonScript harpoon = Instantiate(harpoon_00001,transform.position, Quaternion.identity) as harpoonScript;
 // Assuming harpoon prefab already facing to right
 
 if (facingRight) {
   // Maybe, not required
   harpoon.transform.eulerAngles = new Vector3(0f, 0f, 180f); // Face backward
   Vector3 theScale = harpoon.transform.localScale;
   theScale.y *= -1;
   harpoon.transform.localScale = theScale; // Flip on y axis
 }
 
 
     }
 
 }
 }

harpoonScript.cs (script)

 using UnityEngine;
 using System.Collections;
 
 public class harpoonScript : MonoBehaviour {
 
 // Public variable 
 public int speed = 6;
 private Rigidbody2D r2d;
 
 // Function called once when the bullet is created
 void Start () {
     // Get the rigidbody component
     r2d = GetComponent<Rigidbody2D>();
 
     // Make the bullet move upward
     float ySpeed = 0;
     float xSpeed = -8;
 
     Vector2 newVelocity = new Vector2(xSpeed, ySpeed);
 r2d.velocity = newVelocity;
 
 }
 
 void OnTriggerEnter2D(Collider2D other) //hero hits side of enemy
     {
            
             Destroy(other.gameObject.GetComponent<Collider2D>()); //Remove collider to avoid audio replaying
             other.gameObject.GetComponent<Renderer>().enabled = false; //Make object invisible
             Destroy(other.gameObject, 0.626f); //Destroy object when audio is done playing, destroying it before will cause the audio to stop
   
 }
 
 }