NullReferenceException

This may be a simple fix due to the error being thrown is a NullReferenceException. I am unable though to find where the actual error is in the code where the compiler thinks the error occurs. The two Scripts that are being used are from STRING Augmented Reality so the C# scripts are a little new to me.

I have marked the error with Asterisks to show the line of code where the error is.

Hopefully someone here can help! Thanks

public class DrawingUI : MonoBehaviour {
public Camera mainCamera;
	public Camera stringCamera;
	private bool isMainCameraEnabled = false;
	public bool isStringCameraEnabled = false;

// Use this for initialization
	void Start () {
		mainCamera.enabled = true;
		isMainCameraEnabled = true;
		
		isStringCameraEnabled = false;
		stringCamera.enabled = false;
	
	}
	
	// Update is called once per frame
	void Update () {
		if(isMainCameraEnabled == true)
		{
		mainCamera.enabled = true;
		stringCamera.enabled = false;
		}
		
		if(isStringCameraEnabled == true)
		{
		stringCamera.enabled = true;
		mainCamera.enabled = false;
		}
	
	}
	
	void OnGUI(){
	infoWindow = GUI.Window(1,materialsWindow, materialsWindowFunc, "Material Window");


	
	GUILayout.BeginArea(new Rect (Screen.width / 2 - backButtonWidth /2, Screen.height / 2 - backButtonHeight/2 + 900 - backButtonSpacing,
							 backButtonWidth, backButtonHeight));
		
		if(GUILayout.Button("View Model", GUILayout.Height(backButtonSpacing)))
		{
			isStringCameraEnabled = true;
			isMainCameraEnabled = false;
			cameraCentricARManager.StartString();
			
			
			
			//Application.LoadLevel("StringUI");	
		}
		
		if(GUILayout.Button("Back to Main Menu", GUILayout.Height(backButtonSpacing)))
		{
			Application.LoadLevel("MainMenuUI");
		}
	
	GUILayout.EndArea();
}

public  class CameraCentricARManager : MonoBehaviour {
	public GameObject[] rootObjects;
	public bool fullscreen = false;
	public float alignment = 0.5f;
	public bool reorientBranding = true;
	float[] lastSpottedTimes;
	
	public StringWrapper stringWrapper;
	
	 public void StartString() 
	{
		// Initialize String
		stringWrapper = new StringWrapper(null, camera, reorientBranding, fullscreen, alignment);

		// Load some image targets
		for (uint i = 0; i < rootObjects.Length; i++)
		{
			stringWrapper.LoadImageMarker("Marker " + (i + 1), "png");
		}

		// Hide all rootObjects
		// Also, set them as children of the camera;
		// This is to more easily position them relative to the camera later
		for (uint i = 0; i < rootObjects.Length; i++)
		{
			rootObjects*.SetActiveRecursively(false);*

_ rootObjects*.transform.parent = transform;_
_
}*_

* // Allocate array to track the last time each marker was spotted*
* lastSpottedTimes = new float[rootObjects.Length];*

* // Prevent the iOS keyboard from introducing an unwanted*
* // black frame when rotating*
* iPhoneKeyboard.autorotateToLandscapeLeft = false;*
* iPhoneKeyboard.autorotateToLandscapeRight = false;*
* iPhoneKeyboard.autorotateToPortrait = false;*
* iPhoneKeyboard.autorotateToPortraitUpsideDown = false;*
* }*

* void CreateWrapper()*
* {*
_ ******** //Error occurs here, Located in StringWrapper Update Method ********_
_
// Perform marker image tracking
_
_
uint markerCount = stringWrapper.Update();*_

* // Handle detected markers*
* for (uint i = 0; i < markerCount; i++)*
* {*
* // Fetch tracker data for this marker*
* StringWrapper.MarkerInfo markerInfo = stringWrapper.GetDetectedMarkerInfo(i);*

* if (markerInfo.imageID < rootObjects.Length && rootObjects[markerInfo.imageID] != null)*
* {*
* rootObjects[markerInfo.imageID].transform.localPosition = markerInfo.position;*
* rootObjects[markerInfo.imageID].transform.localRotation = markerInfo.rotation;*
* lastSpottedTimes[markerInfo.imageID] = Time.time;*

* if (!rootObjects[markerInfo.imageID].active)*
* {*
* rootObjects[markerInfo.imageID].SetActiveRecursively(true);*
* }*
* }*
* }*

* // Deactivate rootObjects for lost markers*
* for (uint i = 0; i < rootObjects.Length; i++)*
* {*
_ if (rootObjects*.active)
{
if (Time.time - lastSpottedTimes > 2)
{
// Marker has been out of view for a while; Deactivate it*

rootObjects*.SetActiveRecursively(false);
}
else if (Time.time != lastSpottedTimes)
{
// Marker wasn’t spotted this frame; Hide it*

rootObjects*.transform.localPosition = new Vector3(1000000, 0, 0);
}
}
}
}
}
This is the method from StringWrapper where the error happens
public uint Update()
{
if (wasInitialized)
{
if (isMobile)
{
if (Screen.orientation != currentOrientation && Time.frameCount > 1)
{
ApplyViewSettings();*_

* MobileWrapper.String_SetProjectionAndViewport(_camera.projectionMatrix, camera.rect, (int)(currentOrientation = Screen.orientation), reorientBranding);*
* }*

* markerCount = MobileWrapper.String_GetData(markerInfo, maxMarkerCount);
_ }
else*

* {
if (DesktopWrapper.IsNewFrameReady())
{
DesktopWrapper.ProcessFrame((uint)videoTexture.GetNativeTextureID(), 0);
markerCount = DesktopWrapper.GetDataQuaternionBased(markerInfo, maxMarkerCount);
videoPlaneObject.active = true;
}
}
}
else*

* {
// Write dummy output data*

* markerCount = 1;
markerInfo[0].DummyData();
}*_

* return markerCount;*
* }*

When is StartString called? My guess is either never, or after this Update function is called. Make sure it is called before that Update. Either call it Start or call it FROM Start, or edit the script execution order to put that class first.