C# OnTriggerEnter NullReferenceException

Hello,

I'm using the ZDK from Zigfu for Game Project on my study. I took the standard avatar and attached two colliders (sphere colliders with rigid body) to the hands. One for the left hand, and one for the right hand. I build some (24) box colliders around the avatar, to track if the hands are entering a new area. Every box has an "id" from 0 to 23 an the following script:

 using UnityEngine;
 using System.Collections;
 
 public class handsTracking : MonoBehaviour {
     
     public int id;
     
     void OnTriggerEnter (Collider other) {
         if(other.tag == "handsLeftTracking"){
             print("leftHand entered: " + id);
             playerCommunication.activity += 0.1f;
             playerCommunication.handsLeft[id] = true;
         }
         else if (other.tag == "handsRightTracking"){
             print("rightHand entered: " + id);
             playerCommunication.activity += 0.1f;
             playerCommunication.handsRight[id] = true;
         }
     }
     void OnTriggerExit (Collider other) {
         if(other.tag == "handsLeftTracking"){
             print("leftHand exited: " + id);
             playerCommunication.activity += 0.01f;
             playerCommunication.handsLeft[id] = false;
         }
         else if (other.tag == "handsRightTracking"){
             print("rightHand exited: " + id);
             playerCommunication.activity += 0.01f;
             playerCommunication.handsRight[id] = false;
         }
     }
 }

In another script on the player I want to use these collisions. The handsTracking.cs should only edit the values in the playerCommunication.cs script:

 using UnityEngine;
 using System.Collections;
 
 public class playerCommunication : MonoBehaviour {
     
     public static bool[] handsRight;
     public static bool[] handsLeft;
     public static float activity;
     public float fallback;
     
     // Use this for initialization
     void Awake () {
         activity = 0.0f;
     }
     
     // Update is called once per frame
     void Update () {
         if((activity - fallback * Time.deltaTime) >= 0.0f){
             activity -= fallback * Time.deltaTime;
         }
     }
     
     void OnGUI () {
         GUI.Box (new Rect (10,200,150,20), "Activity: " + activity);    
     }
 }

This works fine, so far. But I'm getting the following exceptions:

NullReferenceException: Object reference not set to an instance of an object handsTracking.OnTriggerEnter (UnityEngine.Collider other) (at Assets/SeriousGame/Scripts/handsTracking.cs:19)

If I commend out lines where I tried to edit the array, the errors are gone. I tried to call a function in the playerCommunication script, or to handle the arrays in the handsTracking.cs , but nothing works. I don't understand the link between the collision and the array?!