Yield inside onCollision generating unassigned null

UnassignedReferenceException: The variable LITHIUM of CowKiller has not been assigned.
You probably need to assign the LITHIUM variable of the CowKiller script in the inspector.
UnityEngine.Object.Internal_InstantiateSingle (UnityEngine.Object data, Vector3 pos, Quaternion rot) (at C:/BuildAgent/work/d63dfc6385190b60/artifacts/EditorGenerated/UnityEngineObject.cs:74)
UnityEngine.Object.Instantiate (UnityEngine.Object original, Vector3 position, Quaternion rotation) (at C:/BuildAgent/work/d63dfc6385190b60/artifacts/EditorGenerated/UnityEngineObject.cs:84)
CowKiller+$spawnCoins$163+$.MoveNext () (at Assets/Standard Assets/Scripts/Enemy/CowKiller.js:311)

That’s the error I’m getting. I’ve double and triple checked that it’s assigned. It is.
I even made a second object, and checked to see

if(Helium == null)
Helium = heliumChecker;

I added the atoms to the resources folder, and assigned them into the inspector from there.
Same error.

This began to occur when I moved takeDamage into a coroutine, it is the calling function of spawncoins…

function spawnCoins()
  if(PlayerScores.health > 999)
	  coinsToSpawn += bonusJunk;
	  //while(Spawned > 0)
	  if(HYDROGEN == null )
	  HYDROGEN = hydrogenChecker ;
	for(var icount : int = 0; icount < coinsToSpawn;icount++ )
	 elemRand = Random.Range(0,1000);
	if(elemRand <= 700)
	gren = HYDROGEN;
	Projectile = HYDROGEN;
	if(elemRand > 700 && elemRand <= 960)
	gren = HELIUM;
	Projectile = HELIUM;
	 	if(elemRand > 960)
	gren = LITHIUM;
	Projectile = LITHIUM;
    gren = Instantiate(Projectile,Spawn.position,transform.rotation);
    gren.transform.position.z = 100;
      gren.rigidbody.transform.forward =    
    yield WaitForSeconds(0.05);

NOW, IF I remove the yield statement, it fixes, but the stuff comes out wrong…

What am I missing

Once I installed the .net 452 update that repaired mono, it made visible about 7 copies of the script on that root object, AFTER planting a new prefab in place. Appears to be an extra default instance for each time I hit “apply” under the prefab…

So, if this happens to you and you don’t see a duplicate script on the object, AND you’re getting random errors from mono saying it’s not a valid project file and so on.

Update .NET framework.
Delete the object and add a fresh copy of the prefab to the scene.
Hit, revert
it should then show the duplicate scripts that it attached. Remove the copies of the script, hit apply, and voila, it works!