TextTracker always null

I'm trying to set up the region of interest using the SetRegionOfInterest from TextTracker class and having a problem with the TextTracker initialization. 

TextTracker textTracker = (TextTracker)TrackerManager.Instance.GetTracker<TextTracker>(); //return null

Rect regionOfInterestTracking =  new Rect(Screen.width * 0.1f, Screen.height * 0.1f,

	                                  Screen.width * 0.8f, Screen.height * 0.4f);

Rect regionOfInterestDetection = new Rect(Screen.width * 0.2f, Screen.height * 0.2f,

	                                  Screen.width * 0.6f, Screen.height * 0.2f);

textTracker.SetRegionOfInterest(regionOfInterestDetection, regionOfInterestTracking);

But it always returns a null reference error, what would be the possible approach for solving this?