collider triggers

i'm trying to change a value on a component when the player collides with a cube but keep getting NullReferenceException: Object reference not set to an instance of an object levelup.OnTriggerEnter (UnityEngine.Collider item) (at Assets/levelup.cs:9)

when i use levelPlus1.rToG += (1); in void start it works fine

here is my code so far thanks for any help

      using UnityEngine;
      using System.Collections;

      public class levelup : MonoBehaviour {
  public levelup levelPlus1;

      void OnTriggerEnter (Collider other) {
             
     levelPlus1.rToG += (1);
 }

}