Resources won't load in my scene at runtime in unity 5?

I have implemented a helper script using a youtube example. But when I use the Resource method in my other scripts the compiler gives an null Reference error. What am I doing wrong.

this is my helper script and how the resources are called.

 using UnityEngine;
 using System.Collections;
 
 namespace Helper
 {
 public class Resource
     {
         public static string AnimatorController = "System/PlayerAnimator";
     }
 }

The path according to my scene is Assets/Resources/System/PlayerAnimator.controller

This is how I used in my other scripts to load the resources

     private Animator _animator;
     private RuntimeAnimatorController _animatorController;
 
  _animatorController = Resources.Load(Resource.AnimatorController) as RuntimeAnimatorController;
  _animator.runtimeAnimatorController = _animatorController;

What is the fix for this?