Unity Roguelike Project: Argument Out of Range Exception - unity3d

Question

I'm getting an Argument Out of Range Exception for this script I'm following in a Unity tutorial.
Here's the exception:
ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
System.Collections.Generic.List`1[UnityEngine.Vector3].get_Item (Int32 index) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:225)
BoardManager.RandomPosition () (at Assets/Scripts/BoardManager.cs:75)
And here's the actual script for the project:
using UnityEngine;
using System; //Allows serializable attribute, which modifies how variables appear
using System.Collections.Generic; //Generic allows use of lists
using Random = UnityEngine.Random; //Used because Random class is in both System and Unity engine namespaces
public class BoardManager : MonoBehaviour
{
[Serializable]
public class Count
{
public int maximum;
public int minimum;
public Count (int min, int max) //Assignment constructor for Count to set values of min, max when we declare a new Count
{
minimum = min;
maximum = max;
}
}
public int columns = 8; //8x8 gameboard
public int rows = 8;
public Count wallCount = new Count(5, 9); //Specifies random range for walls to spawn in each level
public Count foodCount = new Count(1, 5); //Random range for food spawns
public GameObject exit; //Variable that holds exit prefab
public GameObject[] floorTiles; //Game Object Arrays hold our different prefabs to choose between
public GameObject[] wallTiles;
public GameObject[] foodTiles;
public GameObject[] enemyTiles;
public GameObject[] outerWallTiles;
private Transform boardHolder; //Manages Hierarchy
private List<Vector3> gridPositions = new List<Vector3>(); //Declares a private list of Vector3s
//Tracks all possible positions on gameboard, and keeps track of whether object has been spawned in that position or not
void InitialiseList()
{
gridPositions.Clear();
for (int x = 1; x < columns - 1; x++) //Nested FOR loops to fill list with each of the positions on our gameboard as a Vector3
{
for (int y = 1; y < rows - 1; y++)
{
gridPositions.Add(new Vector3(x, y, 0f)); //Adds x and y positions to the list
}
}
}
void BoardSetup() //Sets up outer wall and floor of gameboard
{
boardHolder = new GameObject("Board").transform;
for (int x = -1; x < columns + 1; x++)
{
for (int y = -1; y < rows + 1; y++)
{
GameObject toInstantiate = floorTiles[Random.Range(0, floorTiles.Length)]; //Defines a new GameObject
if (x == -1 || x == columns || y == -1 || y == rows)
{
toInstantiate = outerWallTiles[Random.Range(0, outerWallTiles.Length)];
}
GameObject instance = Instantiate(toInstantiate, new Vector3(x, y, 0f), Quaternion.identity) as GameObject;
//Instantiates a new GameObject. Rotation is always the same. Used as GameObject.
instance.transform.SetParent(boardHolder);
}
}
}
Vector3 RandomPosition()
{
try
{
int randomIndex = Random.Range(0, gridPositions.Count);
Vector3 randomPosition = gridPositions[randomIndex]; //THROWING AN EXCEPTION
gridPositions.RemoveAt(randomIndex); //Prevents duplicate position spawns
return randomPosition;
}
catch (Exception ex1)
{
Debug.Log(ex1);
throw;
}
}
void LayoutObjectAtRandom(GameObject[] tileArray, int minimum, int maximum)
{
int objectCount = Random.Range(minimum, maximum + 1);
for (int i = 0; i < objectCount; objectCount++)
{
Vector3 randomPosition = RandomPosition();
GameObject tileChoice = tileArray[Random.Range(0, tileArray.Length)];
Instantiate(tileChoice, randomPosition, Quaternion.identity);
}
}
public void SetupScene(int level) //Called by the GameManager
{
BoardSetup();
InitialiseList();
LayoutObjectAtRandom(wallTiles, wallCount.minimum, wallCount.maximum);
LayoutObjectAtRandom(foodTiles, foodCount.minimum, foodCount.maximum);
int enemyCount = (int)Mathf.Log(level, 2f); //Scales enemy count based on level
LayoutObjectAtRandom(enemyTiles, enemyCount, enemyCount);
Instantiate(exit, new Vector3(columns - 1, rows - 1, 0f), Quaternion.identity);
}
}
I'm assuming that the RandomPositions() function is throwing the exception, though I am unable to find a problem with the code. Does anyone see the problem?