Code only runs when object is selected in the editor

I have been working on a "Diablo Style" inventory system for my game. The whole script is run off of the Canvas object. If I run the game with the Canvas selected in the editor, everything runs smoothly. However, if I select any other object, or build the game, the Inventory doesn't build correctly.

I think that it has something to do with a private array that wont let me initialize it. If I make it public and set the values in the inspector window, It works fine. However, if I make it private and initialize it through the code, the inventory doesn't build.

This is a simplified version of the code:

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

[System.Serializable] public class SlotData { public GameObject slot; public bool isFull;

 public SlotData()
 {
     slot = null;
     isFull = false;
 }

}

[System.Serializable] public class ItemData { public GameObject item; public GameObject icon; public Vector2Int size; public GameObject slot; public GameObject pickup;

 public ItemData()
 {
     item = null;
     icon = null;
     size = Vector2Int.zero;
     slot = null;
     pickup = null;
 }

}

public class InventoryManager : MonoBehaviour { public Vector2Int inventorySize; public Sprite inventoryTexture; public Sprite cursorImage;

 private GameObject inventoryBackground;
 private GameObject inventorySlots;
 private GameObject inventoryItems;
 private GameObject cursor;

 [HideInInspector]
 public GameObject heldItem;
 private Vector2Int heldItemDimensions;
 private int heldItemNumber;
 private GameObject heldItemIcon;

 public ItemData[] items;
 public SlotData[] slots;
 private int numberOfItems;

 private bool toggleBool;
 private bool InventoryIsOpen;

 private GameObject maxSlot;
 private GameObject minSlot;

 void Start () 
 {
     items = new ItemData[inventorySize.x * inventorySize.y];
     slots = new SlotData[inventorySize.x * inventorySize.y];

     CreateInventory(inventorySize, inventoryTexture, inventoryBackground);
     CreateSlots(inventorySize, inventoryTexture);
 }

void CreateInventory (Vector2 size, Sprite texture, GameObject inventory) { inventory = new GameObject("Inventory"); RectTransform back = inventory.AddComponent(); inventory.transform.SetParent(this.transform); back.anchorMin = new Vector2((1 - (size.x / 10)) / 2, (1 - (size.y / 10)) / 2); back.anchorMax = new Vector2(1 - ((1 - (size.x / 10)) / 2), 1 - ((1 - (size.y / 10)) / 2)); inventory.AddComponent().sprite = texture; inventory.GetComponent().type = Image.Type.Sliced; back.anchoredPosition = Vector2.zero; back.sizeDelta = Vector2.zero; inventoryBackground = inventory;

     // Slot Holder
     inventorySlots = new GameObject("Inventory Slots");
     inventorySlots.transform.SetParent(inventory.transform);
     inventorySlots.AddComponent<RectTransform>().anchorMin = Vector2.zero;
     inventorySlots.GetComponent<RectTransform>().anchorMax = Vector2.one;
     inventorySlots.GetComponent<RectTransform>().anchoredPosition = Vector2.zero;
     inventorySlots.GetComponent<RectTransform>().sizeDelta = Vector2.zero;

     // Icon Holder
     inventoryItems = new GameObject("Inventory Icons");
     inventoryItems.transform.SetParent(inventory.transform);
     inventoryItems.AddComponent<RectTransform>().anchorMin = Vector2.zero;
     inventoryItems.GetComponent<RectTransform>().anchorMax = Vector2.one;
     inventoryItems.GetComponent<RectTransform>().anchoredPosition = Vector2.zero;
     inventoryItems.GetComponent<RectTransform>().sizeDelta = Vector2.zero;

     // Cursor
     cursor = new GameObject("Cursor");
     cursor.transform.SetParent(inventory.transform);
     cursor.gameObject.AddComponent<Image>().sprite = cursorImage;
     cursor.GetComponent<RectTransform>().localScale = new Vector2(0.25f, 0.25f);
     cursor.GetComponent<RectTransform>().pivot = new Vector2(0, 1);
 }

 void CreateSlots (Vector2 size, Sprite texture)
 {
     int i = 0; 
     for(int x = 0; x < size.x; x++)
     {
         for(int y = 0; y < size.y; y++)
         {
             i++;
             GameObject slot = new GameObject("Slot " + (x + 1) + ", " + (y + 1));
             slot.transform.SetParent(inventorySlots.transform);
             slot.AddComponent<Image>().sprite = texture;
             slot.GetComponent<Image>().type = Image.Type.Sliced;
             slot.GetComponent<RectTransform>().anchorMin = new Vector2(0.01f + ((0.98f / size.y) * y), 0.01f + ((0.98f / size.x) * x));
             slot.GetComponent<RectTransform>().anchorMax = new Vector2(0.01f + ((0.98f / size.y) * (y + 1)), 0.01f + ((0.98f / size.x) * (x + 1)));

             slot.GetComponent<RectTransform>().anchoredPosition = Vector2.zero;
             slot.GetComponent<RectTransform>().sizeDelta = Vector2.zero;
             slots[i - 1].slot = slot.gameObject;
         }
     }
 }

}

It always seems to stop working at CreateSlots with a NullReferenceException. Did I initialize or construct the arrays wrong?