UnassignedReferenceException. Object makes itself null partway through script?

UPDATED: I've got an unassigned reference exception on a private gameObject named activeGate. It gives me the error when I collide with an object but doesn't show until then. On start the gameObject activeGate has a value and is called just fine, but partway through the code it seems to become null and gives me the error when called.

I'm very lost so any help would be greatly appreciated. Thank you in advance.

Here's the script: (The error is on the first line of the SpawnGate() function)

 using UnityEngine;
 using System.Collections;
 
 public class GateSpawning : MonoBehaviour 
 {
     public GameObject gatePrefab;
     private GameObject activeGate;
     private GameObject secondaryGate;
     private GameObject chosenSpawnPoint;
     public int MAX_SPAWN_POINTS = 41;
 
     private int spawnPoint;
 
     void Start () 
     {
         ChooseSpawn ();
         activeGate = Instantiate (gatePrefab, chosenSpawnPoint.transform.position, chosenSpawnPoint.transform.rotation) as GameObject;
         ChooseSpawn ();
         secondaryGate = Instantiate (gatePrefab, chosenSpawnPoint.transform.position, chosenSpawnPoint.transform.rotation) as GameObject;
         activeGate.GetComponent<GateLogic> ().activate ();
     }
     
 
     void Update () 
     {
         
     }
 
     public void SpawnGate ()
     {
         // Destroys currently active gate
         activeGate.GetComponent<GateLogic> ().destroyGate ();
 
         // Secondary gate becomes active gate and plays its activation animation
         activeGate = secondaryGate;
         activeGate.GetComponent<GateLogic> ().activate ();
 
         ChooseSpawn ();
 
         // Instantiate a new gate and assign it to secondaryGate
         secondaryGate = Instantiate (gatePrefab, chosenSpawnPoint.transform.position, chosenSpawnPoint.transform.rotation) as GameObject;
     }
 
     public void ChooseSpawn ()
     {
         spawnPoint = Random.Range (1, MAX_SPAWN_POINTS);
         chosenSpawnPoint = GameObject.Find ("SpawnPoint" + spawnPoint);
     }
 }