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[i].SetActiveRecursively(false);
             rootObjects[i].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[i].active)
             {
                 if (Time.time - lastSpottedTimes[i] > 2)
                 {
                     // Marker has been out of view for a while; Deactivate it
                     rootObjects[i].SetActiveRecursively(false);
                 }
                 else if (Time.time != lastSpottedTimes[i])
                 {
                     // Marker wasn't spotted this frame; Hide it
                     rootObjects[i].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;
     }