Object reference not set to an instance of an object

Questions & Answers
1

I keep getting this error after I expanded my spawning class.

NullReferenceException: Object reference not set to an instance of an object
Spawner.SpawnEnemy () (at Assets/Scripts/Spawner.cs:23)
SpawnController.startWave () (at Assets/Scripts/SpawnController.cs:47)
SpawnController.waveController () (at Assets/Scripts/SpawnController.cs:29)

Bellow you will find my Enemy Spawn and SpawnManager classes. Any help would be appreciated, this is driving me bonkers.

using UnityEngine;
using System.Collections;

public class Spawner : MonoBehaviour {

public GameObject Enemy;
GameObject[] objs;
int top;

void Start()
{
    objs = GameObject.FindGameObjectsWithTag("Spawn_Point");
    top = objs.Length;
 //      InvokeRepeating("SpawnEnemy", 0.01f, 5.0f);
}

void Update()
{
}

public void SpawnEnemy()
{
    Instantiate(Enemy, RandomCircle(objs[Random.Range(0, top)].GetComponent<Transform>().position, 5), Quaternion.identity);
    SpawnController.numAlive++;
}

Vector3 RandomCircle(Vector3 center, float radius)
{
    float ang = Random.value * 360;
    Vector3 pos;
    pos.x = center.x + radius * Mathf.Sin(ang * Mathf.Deg2Rad);
    pos.y = center.y;
    pos.z = center.z + radius * Mathf.Cos(ang * Mathf.Deg2Rad);
    return pos;
}

}

using UnityEngine;
using System.Collections;

public class SpawnController : Spawner {

    public static int numAlive;
    int round;
    bool active;

	// Use this for initialization
	void Start () {
        numAlive = 0;
        round = 0;
        active = false;
        InvokeRepeating("waveController", 0.01f, 15.0f);
    }
	
	// Update is called once per frame
	void Update ()
    {
	}

    void waveController()
    {
        if(!active)
        {
            Debug.Log("Active now = true");
            active = true;
            startWave();
        }
        else
        {
            if(numAlive == 0)
            {
                Debug.Log(" End round ");
                active = false;
                round++;
            }
        }
    }


using UnityEngine;
using System.Collections;

namespace SpawningFramework
{
    public class Enemy : MonoBehaviour
    {

        public float MaxHealth;
        float health;
        Vector3 pos;


        [HideInInspector]
        public bool alive;

        void Update()
        {
        }

        /// Called when this enemy has been spawned
        void Start()
        {
            health = MaxHealth;
            alive = true;
            pos = transform.position;
            
            InvokeRepeating("move", 0.01f, .2f);

        }

        void move()
        {
           // transform.Translate(0, 0.025f, 0);
        }

        /// Called when the player loses
        public virtual void OnGameOver()
        {
        }

        void OnTriggerEnter(Collider other)
        {
            Debug.Log(other.tag);
            if (other.tag == ("Player"))
            {
                Hurt(54);
//              Destroy(other.gameObject);        
            }
        }

        #region Taking Damage
        /// Hurts the enemy and returns if it dies or not
        public virtual bool Hurt(float damage)
        {
            if (!alive)
                return false;//exit if dead

            health -= damage;//hurt the enemy

            if (health < 0.1)
            {
                alive = false;
                SpawnController.numAlive--;
                Destroy(this.gameObject);
                Destroy(this);
                return true;
            }

            return health < 0.1;//return if this will die
        }


        #endregion
    }
}


    void startWave()
    {
        Debug.Log("Starting wave: " + round);
        for(int i = 0; i < (8 + 2 * round); i++)
        {
            SpawnEnemy();
        }
    }
}
2

in class Spawner :

SpawnController scontroler;
void Start()
{
scontroler =  GetComponent<SpawnController > ();
}
public void SpawnEnemy()
{
scontroler.numAlive++; 
}

the same for class Enemy .