UnassignedReferenceException: The variable flyPrefab of FlySpawner has not been assigned.

[SerializeField]
private GameObject flyPrefab;

[SerializeField]
private int totalFlyMinimum = 12;

private float spawnArea = 25f;

public static int totalFlies;

// Use this for initialization
void Start () {
    totalFlies = 0;
}

// Update is called once per frame
void Update () {

    //While the total number of flies is less than the minimum...
    while (totalFlies < totalFlyMinimum)
    {
        //...then increment the total number of flies...
        totalFlies++;

        //...create a random position for a fly
        float positionX = Random.Range(-spawnArea, spawnArea);
        float positionZ = Random.Range(-spawnArea, spawnArea);

        Vector3 flyPosition = new Vector3(positionX, 2f, positionZ);

        //...and create a new fly.
        Instantiate(flyPrefab, flyPosition, Quaternion.identity);
    }

}