IndexOutOfRangeException: Index was outside the bounds of the array. (wrapper managed-to-managed) System.Object.ElementAddr_4(object,int,int,int)

Hey I am super stuck with my unity game trying to add sound to my characters footsteps :(

https://www.youtube.com/watch?v=Bnm8mzxnwP8 I was following this tutorial and got to the end then when i press play in unity and walk it crashes and gives me this error -

  IndexOutOfRangeException: Index was outside the bounds of the array.
  (wrapper managed-to-managed) System.Object.ElementAddr_4(object,int,int,int)
  TerrainDetector.GetActiveTerrainTextureIdx (UnityEngine.Vector3 position) (at 
  Assets/TerrainDetector.cs:39)
  FootSteps.GetRandomClip () (at Assets/FootSteps.cs:29)
  FootSteps.Step () (at Assets/FootSteps.cs:23)

Here is the code for TerrainDetector:

 using UnityEngine;

 public class TerrainDetector
 {
 private TerrainData terrainData;
 private int alphamapWidth;
 private int alphamapHeight;
 private float[,,] splatmapData;
 private int numTextures;

 public TerrainDetector()
 {
     terrainData = Terrain.activeTerrain.terrainData;
     alphamapWidth = terrainData.alphamapWidth;
     alphamapHeight = terrainData.alphamapHeight;

     splatmapData = terrainData.GetAlphamaps(0, 0, alphamapWidth, alphamapHeight);
     numTextures = splatmapData.Length / (alphamapWidth * alphamapHeight);
 }

 private Vector3 ConvertToSplatMapCoordinate(Vector3 worldPosition)
 {
     Vector3 splatPosition = new Vector3();
     Terrain ter = Terrain.activeTerrain;
     Vector3 terPosition = ter.transform.position;
     splatPosition.x = ((worldPosition.x - terPosition.x) / ter.terrainData.size.x) * ter.terrainData.alphamapWidth;
     splatPosition.z = ((worldPosition.z - terPosition.z) / ter.terrainData.size.z) * ter.terrainData.alphamapHeight;
     return splatPosition;
 }

 public int GetActiveTerrainTextureIdx(Vector3 position)
 {
     Vector3 terrainCord = ConvertToSplatMapCoordinate(position);
     int activeTerrainIndex = 0;
     float largestOpacity = 0f;

     for (int i = 0; i < numTextures; i++)
     {
         if (largestOpacity < splatmapData[(int)terrainCord.z, (int)terrainCord.x, i])
         {
             activeTerrainIndex = i;
             largestOpacity = splatmapData[(int)terrainCord.z, (int)terrainCord.x, i];
         }
     }

     return activeTerrainIndex;
 }

}

and here is the code for FootSteps: using UnityEngine;

 public class FootSteps : MonoBehaviour
 {
 [SerializeField]
 private AudioClip[] stoneClips;
 [SerializeField]
 private AudioClip[] mudClips;
 [SerializeField]
 private AudioClip[] grassClips;

 private AudioSource audioSource;
 private TerrainDetector terrainDetector;

 private void Awake()
 {
     audioSource = GetComponent<AudioSource>();
     terrainDetector = new TerrainDetector();
 }

 private void Step()
 {
     AudioClip clip = GetRandomClip();
     audioSource.PlayOneShot(clip);
 }

 private AudioClip GetRandomClip()
 {
     int terrainTextureIndex = terrainDetector.GetActiveTerrainTextureIdx(transform.position);

     switch(terrainTextureIndex)
     {
         case 0:
             return stoneClips[UnityEngine.Random.Range(0, stoneClips.Length)];
         case 1:
             return mudClips[UnityEngine.Random.Range(0, mudClips.Length)];
         case 2:
         default:
             return grassClips[UnityEngine.Random.Range(0, grassClips.Length)];
     }
     
 }
 }

Hope you can help! It would mean a lot :D