Instantiate ground transform x units from previous ground prefab

Im having a problem trying to figure out an algorithm and how to code getting my ground prefabs to instantiate in a logical order where they dont overlap each other. To start off this is just a prototype infinite runner and the object is to get the ground prefabs to instantiate randomly while the runner runs infinitely to the right. The one code that works is the code from the unity infinite runner tutorial (37:45) but with my ground prefabs they overlap each other. I've tried changing the values testing multiple values and it doesn't seem to change much and when it does its unplayable with the platforms spread too far apart.

I tried modifying the code to spread them farther apart but it actually doesn't compile. Its a nullreference error but im just completely stuck on it. I'll provide the code that ran but overlapped below too. I attacked a picture of the overlapping


Code that I modified

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class SpawnScript : MonoBehaviour {
5.  
6.     public GameObject[] obj;
7.     private Transform transform;
8.     public float spawnMin = 1f;
9.     public float spawnMax = 2f;
10.     public float wait = 1f;
11.  
12.     IEnumerator Spawn(Collider2D other)
13.     {
14.         Instantiate (obj [Random.Range (0, obj.Length)], transform.position, Quaternion.identity);
15.         if(other.CompareTag("Ground"))
16.         {
17.             yield return new WaitForSeconds(wait);
18.             Invoke ("Spawn", Random.Range(spawnMin, spawnMax));
19.         }
20.         else
21.             Invoke ("Spawn", Random.Range(spawnMin, spawnMax));
22.     }
23.  
24.     void OnTriggerEnter2D(Collider2D other)
25.     {
26.         StartCoroutine (Spawn (other));
27.     }
28. }

Code that ran

Code (csharp):

1.  
2. using UnityEngine;
3. using System.Collections;
4.  
5. public class SpawnScript : MonoBehaviour {
6.  
7.     public GameObject[] obj;
8.     public float spawnMin = 1f;
9.     public float spawnMax = 2f;
10.  
11.     void Start()
12.     {
13.         Spawn ();
14.     }
15.  
16.     void Spawn()
17.     {
18.         Instantiate (obj [Random.Range (0, obj.Length)], transform.position, Quaternion.identity);
19.         Invoke ("Spawn", Random.Range(spawnMin, spawnMax));
20.     }
21. }
22.  

 

Are the objects only created left-to-right? (AKA can I rely on the fact that the new object will HAVE to be on the right of the previous object?)

 

yes they are only created on the right side of the screen and move left. I have a OutOfBoundsDestroyer that destroys the prefab ground piece shortly after the camera passes