Error with object referencing

Unity is giving me the following error:
NullReferenceException: Object reference not set to an instance of an object
SupplyCrash.Update () (at Assets/SupplyDrop/Supply/SupplyCrash.cs:18)
I know the issue is with the "smokeClone.transform.position = droppedSupply.transform.position;" part of the code (or I believe it is), but I have no idea how to correct it. Though the error is coming up, everything works as expected when I test the game. Here is the exact code I am using:

Code (CSharp):

1.     public GameObject crashSmoke;
2.     public GameObject droppedSupply;
3.     GameObject smokeClone;
4.  
5.     bool hasCollided = false;
6.  
7.     private void Update()
8.     {
9.         smokeClone.transform.position = droppedSupply.transform.position;
10.     }
11.  
12.     private void OnCollisionEnter(Collision collision)
13.     {
14.         if (!hasCollided)
15.         {
16.             smokeClone = Instantiate(crashSmoke, transform.position, transform.rotation);
17.             hasCollided = true;
18.         }
19.        
20.     }

 

"smokeClone" is assigned in the OnCollisionEnter() function but you are trying to reference it every frame in the Update() function.

Consider checking whether it's null first:

Code (CSharp):

1. void Update()
2. {
3.     if(smokeClone != null)
4.     {
5.         smokeClone.transform.position = droppedSupply.transform.position;
6.     }
7. }