Why would AssetBundle.LoadAsync give NullReference only sometimes?

I have an asset bundle that I have loaded for my application. Then only sometimes (and I cannot pin down the reason its very random) I get a null reference exception like so:

 12-24 14:44:28.859: I/Unity(2085): NullReferenceException: Object reference not set to an instance of an object
 12-24 14:44:28.859: I/Unity(2085):   at TagTrackableEventHandler+<loadModelAsync>c__Iterator0.MoveNext () [0x00000] in <filename unknown>:0 

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The only place where I would be moving with iterators (i think) is when getting something from an asset bundle.

Here is the rest of the function of that script private IEnumerator loadModelAsync() {

     AssetBundle bundle = universalHolder.GetComponent<LoadAssetBundle>().returnAssetBundle();
     if(bundle != null && bundle.Contains(this.mTrackableBehaviour.TrackableName) && m3dModel == null && !MemoryCleaner.unloading)
     {
         startAnimation();
         model = bundle.LoadAsync(this.mTrackableBehaviour.TrackableName, typeof(GameObject));
             
         isModelLoading = true;
         yield return model;
         if(model.asset != null)
         {
             if(currentStatus == TrackableBehaviour.Status.TRACKED || 
                     currentStatus == TrackableBehaviour.Status.DETECTED)
             {
     
                 m3dModel = Instantiate(model.asset) as GameObject;
                 yield return null;
                     
                 //Adds model to memory cleaner buffer
                     MemoryCleaner.addModelRender(mTrackableBehaviour.TrackableName);
                     
                 //Makes sure model always follows Image Target
                     m3dModel.transform.parent = this.transform;
                     
                     //Sets ratio based on largest dimension
                 ratio = desiredSize/(Mathf.Max(m3dModel.renderer.bounds.size.x,m3dModel.renderer.bounds.size.y, m3dModel.renderer.bounds.size.z));
                     
                 baseScale.x = ratio;
                 baseScale.y = ratio;
                 baseScale.z = ratio;
                     m3dModel.transform.localScale = baseScale;
     
                     //Calculates y position to simulate standing on image target.
                 float yPosition = ((m3dModel.renderer.bounds.size.y /(m3dModel.transform.lossyScale.y / m3dModel.transform.localScale.y ))/2);
     
                 yPosVector.y = yPosition;
                     m3dModel.transform.localPosition = yPosVector;
                     
                     //Sets base rotation (180 degrees around y axis)
                     m3dModel.transform.localRotation = Quaternion.AngleAxis(baseRotation, Vector3.up);
     
                     m3dModel.renderer.enabled = true;
     
                 m3dModel.layer = LayerMask.NameToLayer("ARLayer");
                     
                     //Add a collider for touch actions
                     m3dModel.AddComponent<BoxCollider>();
                 m3dModel.name = mTrackableBehaviour.TrackableName;
                     
                     //Adds the ModelBehaviour Component to react to touches.
                     m3dModel.AddComponent<ModelBehavior>();
     
                     m3dModel.GetComponent<ModelBehavior>().setModelRatio(ratio);
                     
                 isModelLoading = false;
                     
                     //Clears releases memory of model loaded from asset bundle;
                 model = null;
                     
             }
         }
     }
         
     //Stops targeting Animation
     targetingAnimation.stop();
     //Hides Animaiton
     scanTargeting.hidden = true;
 
     //Stops position update of animation.
     isAnimPlaying = false;
         
 }

Also the weird thing is it doesn't always happen. Sometimes the same model (a bear in this case) will load up 10 times in a row without any problems then i will get the above error and I will not be able to instantiate it again.