How to get variables of a behavior tree with RAIN AI

Hello there ! I search how to use the variable of my behavior tree. I have a form variable called "varVisual", when nothing is detected the variable is null, and when something is detected the variable is'nt null. The sensor is a visual sensor. I want to know in my script when the variable is null or not. I tried a lot of things, like this :

     void Awake()
     {
         _aiRig = GetComponentInChildren<AIRig> ();
                visualSensor = _aiRig.AI.Senses.GetSensor ("eye");
 
     }
 
     // Use this for initialization
     void Start ()
     {
         if (visualSensor == null) {
             Debug.Log ("visualSensor == null");
         } else if (visualSensor != null) {
             Debug.Log("visualSensor != null");    
     }
 
     

How could I know the state of the constraint in my behavior Tree (RUNNING or FAILURE) or what varVisual is equals (null or != null) in my script.

Thanks !