Unity 3D(C#) - How to connect more than two box colliders used as triggers

Question

The current error : NullReferenceException: Object reference not set to an instance of an object BackboardTrigger.OnTriggerEnter (UnityEngine.Collider altCollider) (at Assets/Scripts/BackboardTrigger.cs:10)

I believe my question is different from, What is a NullReferenceException, and how do I fix it?, because it requires a narrow answer. The other post provides a broad scope of what a NullReferenceException is while I need to know how to connect more than two triggers.

I'm a novice in C# and I'm attempting to recreate a simple basketball game. As of now I'm trying to trigger a score for when the ball hits the backboard first and sequentially enters the hoop. The tutorial currently teaches how to trigger a score for when the ball begins to enter the hoop and reaches near the bottom of it.

Right now when the ball enters the hoop through the first trigger(PrimaryTrigger) and sequentially through the second(SecondaryTrigger) a score is triggered.

I'm trying to cause a different scoring action for when the ball hits the backboard first.

My PrimaryTrigger Script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;

 public class PrimaryTrigger : MonoBehaviour {

     void OnTriggerEnter(Collider collider)
     {
         SecondaryTrigger trigger = GetComponentInChildren<SecondaryTrigger>();
         trigger.ExpectCollider(collider);
     }
 }

SecondaryTrigger Script:

using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;

 public class SecondaryTrigger : MonoBehaviour {

     Collider expectedCollider;
     Collider possibleCollider;

     public void ExpectCollider(Collider collider)
     {
         expectedCollider = collider;
     }

     public void PossibleCollider(Collider altCollider)
     {
         possibleCollider = altCollider;
     }

     void OnTriggerEnter(Collider otherCollider)
     {

         if(otherCollider == expectedCollider && otherCollider == possibleCollider)
         {
             ScoreKeeper scoreKeeper = FindObjectOfType<ScoreKeeper>();
             scoreKeeper.IncrementScore(1);
         }

         else if(otherCollider == expectedCollider)
         {
             ScoreKeeper scoreKeeper = FindObjectOfType<ScoreKeeper>();
             scoreKeeper.IncrementScore(2);
         }

     }
 }

BackboardTrigger Script:

using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;

 public class BackboardTrigger : MonoBehaviour {

     void OnTriggerEnter(Collider altCollider)
     {
         SecondaryTrigger newTrigger = GetComponent<SecondaryTrigger>();
         newTrigger.PossibleCollider(altCollider);
     }
 }

Any form of help would be greatly appreciated. My first game in Unity.