Instatiated GameObject won't move even though it has a rigidbody

I have a prefab I instantiated as a game object and the game object has a rigidbody that is instantiated on a when the user presses a key then it should have force added to it when the triggered animation reaches a certain frame.Instead I just get the nullrefrenceexception error when I try to add force. Can someone please help me?

Here is my script: using UnityEngine; using System.Collections;

 public class RoniCombat : MonoBehaviour 
 {
     //declare the bat prefab to be added in inspector
     public GameObject bat;
     //declare the knife prefab to be added in inspector
     public GameObject knife;
 
     //declare the spawn point of the knife prefab to be added in inspector
     public GameObject knifeSpawn;
 
     //force to add to knife when it is thrown
     public float force = 50;
 
     //declare the Rigidbody that will be attached to the instatiated knife
     Rigidbody knifeInstance;
 
     //declare the animation
     Animator anim;
 
     //bool to see if the bat should be active
     bool usingBat;
 
     void Start () 
     {
         //grab the animator
         anim = GetComponent<Animator> ();
 
         //check to see if the player is using the bat...
         if (bat.activeSelf) {
         //if so enable the bat in the scene
             usingBat = true;
             bat.SetActive(true);
         }
         //otherwise...
         else 
             
         {
         //disable the bat and use knives
             usingBat = false;
             bat.SetActive(false);
         }
     }
     
     void Update () 
     {
         //if the player has pressed the button to switch weapons...
         if(Input.GetButtonDown("Switch"))
             {
             //toggle the bat
             bat.SetActive (!bat.activeSelf);
             }
 
         //if they press the swing button and the bat is in the scene...
         if (Input.GetButtonDown ("Swing") &&  bat.activeSelf)
         {
             //swing the bat
             anim.SetBool ("Swing", true);
         }
         //otherwise
         else 
         {
             //don't swing the bat
             anim.SetBool ("Swing", false);
         }
 
         //if the throw button is pressed...
         if (Input.GetButtonDown ("Throw") && !bat.activeSelf)
         {
             //call the spawn knife method
             spawnKnife ();
             //perform the throwing animation
             anim.SetBool ("Throw", true);
 
         }
 
         //otherwise...
         else 
         {
             //don't perform the throwing animation
             anim.SetBool ("Throw", false);
         }
 
 
     }
 
     //create the spawnKnife() method
       void spawnKnife ()
     {
         //assign the knifeInstance rigidbody a value
         knifeInstance = Instantiate(knife, knifeSpawn.transform.position, (Quaternion.Euler(0,-90,0))) as Rigidbody;
 
     }
 
     //declare the throwKnife method which is called as an animation event
      void throwKnife()
     {
         //propel the knife forward
         knifeInstance.AddForce (0, 0, force);
 
     }
 
 }

The animation clearly has the animation event attached. is it because knifeInstance isn't assigned a value inside of the throwKnife() Method?