Fly not being "destroyed"; (potentially related) "UnassignedReferenceException" error

Hello! Thanks for this great series. I really appreciate you taking the time to put together a great template program to build from start-to-finish.

In short, my fly isn't being "destroyed" upon contact with my frog. I have watched and reviewed your FlyMovement and FlyPickup videos, but I don't seem to have overlooked anything. I speculate that the following compiler error is related, but cannot confirm whether this is the case.

The full text of the error is: UnassignedReferenceException: The variable center of FlyMovement has not been assigned. You probably need to assign the center variable of the FlyMovement script in the inspector. UnityEngine.Transform.get_position () (at C:/buildslave/unity/build/artifacts/generated/common/runtime/UnityEngineTransformBindings.gen.cs:26) FlyMovement.Update () (at Assets/Scripts/FlyMovement.cs:20)

Have I missed something? Have I somehow failed to "assigned the center variable of the FlyMovement script in the inspector"? I have pasted my FlyMovement.cs text below.

Thanks, in advance, for any help or insight that you can provide!

using UnityEngine;
using System.Collections;

public class FlyMovement : MonoBehaviour {
    [SerializeField]
    private Transform center;

    private float flySpeed;

    // Use this for initialization
    void Start () {
        //the following "range" method picks a value between the minimum float (300) and the maximum float (700)
        flySpeed = Random.Range (300f, 700f);


    }

    // Update is called once per frame
    void Update () {
        transform.RotateAround (center.position, Vector3.up, flySpeed * Time.deltaTime);



    }
}