erros and bugs

I'm trying to make 2d roguelike game but somehow the image looks scary. here are the error's that I received from the console.

/Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/List.cs:633) BoardManager.RandomPosition () (at Assets/Script/BoardManager.cs:69) BoardManager.LayoutObjectAtRandom (UnityEngine.GameObject[] tileArray, Int32 minimum, Int32 maximum) (at Assets/Script/BoardManager.cs:79) BoardManager.SetupScene (Int32 level) (at Assets/Script/BoardManager.cs:90) GameManager.InitGame () (at Assets/Script/GameManager.cs:30) GameManager.Awake () (at Assets/Script/GameManager.cs:25)

all of my sprites are 254x254.

 using UnityEngine;
 using System;
 using System.Collections.Generic;
 using Random = UnityEngine.Random;
 
 public class BoardManager : MonoBehaviour 
     {
         [Serializable]
         public class Count
         {
             public int minimum;
             public int maximum;
         
             public Count (int min, int max)
             {
                 minimum = min;
                 maximum = max;
             }
     }
 
         public int columns = 8;
         public int rows = 8;
         public Count wallCount = new Count (5,9);
         public Count foodCount = new Count (1,5);
         public GameObject exit;
         public GameObject[] floorTiles;
         public GameObject[] wallTiles;
         public GameObject[] foodTiles;
         public GameObject[] enemyTiles;
         public GameObject[] outerWallTiles;
 
         private Transform boardHolder;
         private List <Vector3> gridPositions = new List<Vector3>();
 
         void InitialiseList()
         {
             gridPositions.Clear ();
             for (int x =1; x < columns - 1; x++)
             {
                 for (int y = 1; y < rows - 1;y++)
                 {
                     gridPositions.Add(new Vector3(x,y,0f));
                 }
             }
         }
 
     void BoardSetup()
     {
             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)];
                     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;
 
                     instance.transform.SetParent(boardHolder);
 
                 }
             }
     }
     
     Vector3 RandomPosition()
         {
             int randomIndex = Random.Range(0,gridPositions.Count);
             Vector3 randomPosition = gridPositions [randomIndex];
             gridPositions.RemoveAt (randomIndex);
             return randomPosition;
         }
 
         void LayoutObjectAtRandom(GameObject[] tileArray, int minimum, int maximum)
         {
             int objectCount = Random.Range (minimum, maximum + 1);
             for (int i = 0; i < objectCount; i++)
             {
                 Vector3 randomPosition = RandomPosition();
                 GameObject tileChoice = tileArray[Random.Range (0, tileArray.Length)];
                 Instantiate (tileChoice, randomPosition, Quaternion.identity);
             }
         
         }
 
         public void SetupScene (int level)
         {
             BoardSetup ();
             InitialiseList ();
             LayoutObjectAtRandom(wallTiles, wallCount.minimum, wallCount.maximum);
             LayoutObjectAtRandom(foodTiles, foodCount.minimum, foodCount.maximum);    
             int enemyCount = (int)Mathf.Log (level, 2f);
             LayoutObjectAtRandom(enemyTiles, enemyCount, enemyCount);
             Instantiate (exit, new Vector3 (columns - 1, rows - 1, 0f), Quaternion.identity);
 
         }
     }
 
 

also I think my code is perfectly fine.