Adding Force to an instantiated object when it spawns?

Hi, having trouble wrapping my head around achieving this:

I'm making a game where there are 4 spawn points on the right side of the screen.

I have an array of objects(1 in it at the moment) I also have an array of spawnpoints (4)

My aim is to have the object move to the left side of the screen slowly until it reaches a trigger.

My current script is below.

My objects instantiate fine, and spawn in the right spots (grabs a random object from the "Junk" Gameobject[] array, spawns it in one of the 4 spawnpoints from the SpawnPoints[] Transform array.

How do I manipulate the spawned object to move in said motion?

I figure I need to make it a rigidbody, and use rigidbody.addforce, but i'm missing something!

The error message is:

SpawnItems.SpawnJunk() (Line 41): NullReferenceException: Object Reference not set to an instance of an object.

What am I forgetting to pass?

"JunkInstance.AddForce (SpawnPoints[spawnindex].forward * 800);"

Junkinstance is our instanced rigidbody. AddForce demands a vector3, Spawnpoints is our transform array, and spawnindex is a stored transform. .forward returns a vector3, so surely I'm not missing anything? 0_O. This isn't making sense to me :|

 using UnityEngine;
 using System.Collections;
 
 public class SpawnItems : MonoBehaviour {
 
 
     // indexes Junk into an array
     // indexes SpawnPoints into an array
     // Spawns random Junk obj from a random SpawnPoint.
     // Assign object to move down the lane it spawned in.
 
     public Transform[] SpawnPoints; // array of spawnpoints, linked with transform in editor. this is also beginning of our "lanes".
     public float SpawnTime = 1.5f; // time between spawn "waves"
     public GameObject[] Junk; // Array of Junk Objects
 
     // Use this for initialization
     void Start () {
     
         InvokeRepeating ("SpawnJunk", SpawnTime, SpawnTime);
         // invokes SpawnJunk method after $SpawnTime seconds, then again every $SpawnTime seconds.
 
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     void SpawnJunk()
     {
         int spawnindex = Random.Range (0, SpawnPoints.Length);
             // set index number of the array randomly
 
         int objectIndex = Random.Range (0, Junk.Length);
             // object Index = random number in range of between 0 & GameObject[] Junk.Length
 
         Rigidbody JunkInstance;
         JunkInstance = Instantiate (Junk[objectIndex], SpawnPoints [spawnindex].position, SpawnPoints [spawnindex].rotation) as Rigidbody;
         // Instantiates a random Junk obj to a random spawnpoint, following the spawnpoint's rotation.
 
         JunkInstance.AddForce (SpawnPoints[spawnindex].forward * 800);
         // BROKEN!
     }
 }