Game Manager can't decide what the instance of the object is despite just making an instance of it

Problem: Unity is giving me this error:

NullReferenceException: Object reference not set to an instance of an object GameMangerController.Start () (at Assets/Scripts/GameMangerController.cs:17)

And that problem is with this code, (bolded line 17):

 public class GameMangerController : MonoBehaviour {

     public GameObject ballPrefab;
     private Ball currentBall;


     // Use this for initialization
     void Start()
     {
         GameObject ballInstance = Instantiate(ballPrefab, transform);

         currentBall = ballInstance.GetComponent<Ball>();
         **currentBall.transform.position = Vector3.zero;**

     }
 }

with my relevant Ball.cs being:

 public class Ball : MonoBehaviour {
     public float minXSpeed = .5f;
     public float maxXSpeed = 1.5f;
     public float minYSpeed = 0f;
     public float maxYSpeed = 1.5f;

     Rigidbody2D ballRigidBody;

     // Use this for initialization
     void Start () {
         ballRigidBody = GetComponent<Rigidbody2D>();
         ballRigidBody.velocity = new Vector2(
             Random.Range(minXSpeed,maxXSpeed) * (Random.value > 0.5f ? -1 : 1),
             Random.Range(minYSpeed, maxYSpeed) * (Random.value > 0.5f ? -1 : 1)
             );
     }
 }

Attempt at Solution:

And I have checked that all my filenames are correct and that I dragged the prefab to the correct public area on the GameManager. In the scene, there is no ball object, the only ball object is the one instantiated by the GameManager.

Rewrote the code for GameManager and Ball a few times to ensure there were no typos.

Notes: I am pretty new to C# and Unity in general, so I've been following tutorials on how to get started. This same issue presented itself to me in a previous project, but I was never able to fix it there either.