Creating a spawn controller script in Unity - NullReferenceException: Object reference not set

Question

I'm trying to use a 'spawnController' script to randomly instantiate gameObjects based on a predefined set of spawnpoints and destinations.

My 'SpawnSingle' script works, but I'm having difficulty creating the controller script.

Here is the 'SpawnController' below.

using UnityEngine;
using System.Collections;

public class SpawnController : MonoBehaviour {

public GameObject[] Enemies;
public Transform[] SpawnPoint;
public Transform[] PitStop;
public Transform[] Destination;
public int spawnCounter;

private Transform currentDestination;
private NavMove moveScript;
private GameObject newEnemy;
private SpawnSingle newSpawn;

// Use this for initialization
void Start()
{
    // To-Do: Build a function to randomise the route and spawn at intervals
    spawnCounter = 0;
    Spawn();

}

void Spawn()
{
    SpawnSingle.spawn.SpawnEnemy(Enemies[spawnCounter], SpawnPoint[spawnCounter], PitStop[spawnCounter], Destination[spawnCounter]);
// THIS DOESN'T work either?
// newSpawn.spawn.SpawnEnemy(Enemies[0], SpawnPoint[0], PitStop[0], Destination[0]);
}
}

The error is occuring in Spawn() above.

NullReferenceException: Object reference not set to an instance of an object SpawnController.Spawn () (at Assets/__Custom/Scripts/SpawnController.cs:29) SpawnController.Start () (at Assets/__Custom/Scripts/SpawnController.cs:22)

And for the 'SpawnSingle'

public class SpawnSingle : MonoBehaviour {

public static SpawnSingle spawn;
public GameObject Enemies;
public Transform SpawnPoint;
public Transform PitStop;
public Transform Destination;

private Transform currentDestination;
private NavMove moveScript;
private GameObject newEnemy;

public void SpawnEnemy(GameObject Enemies2, Transform SpawnPoint2, Transform PitStop2, Transform Destination2)
{
    newEnemy = Instantiate(Enemies, SpawnPoint.position, Quaternion.identity) as GameObject;
    moveScript = newEnemy.GetComponent<NavMove>();

    currentDestination = PitStop;
    moveScript.destination = currentDestination;
}
....
....
....

I've also googled it, and it's a common error... I think I'm missing something really basic here in terms of declaring or using a variable. Tried a few variations and other things that popped up in search but no luck.

I'm quite new at this, so thanks for the help.