Why does my mesh's vertices all set to 0 when I try to access it using Mesh.vertices?

I'm having a really weird problem where all the vertices of my mesh returns Vector3.zero when I try to copy them from Mesh.vertices.

When the GeneratePreviewNoise() function is called, the entire Mesh becomes invisble in the scene because all the vertices have the same location (0, 0, 0).

Why does this happen? o.O

My code currently looks like this:

 using UnityEngine;
 using System.Collections.Generic;
 using System.Linq;
 #if UNITY_EDITOR
 using UnityEditor;
 #endif
 
 public class WorldManager : MonoBehaviour
 {
     public Vector3 noisePos;
 
     [SerializeField] private int m_size;
 
     [SerializeField] private float m_amplitude;
     [SerializeField] private float m_frequency;
     [SerializeField] private float m_lacunarity;
     [SerializeField] private float m_invLacunarity;
     [SerializeField] private int m_octaves;
 
     [SerializeField] private GameObject m_quad;
     [SerializeField] private List<GameObject> m_chunks = new List<GameObject>();
     private Material m_sharedQuadMat;
 
     private void Start()
     {
         //m_sharedQuadMat = m_quad.GetComponent<MeshRenderer>().sharedMaterial;
     }
 
     public void GeneratePreviewNoise()
     {
         m_sharedQuadMat = m_quad.GetComponent<MeshRenderer>().sharedMaterial;
         Texture2D tex = new Texture2D(m_size, m_size);
 
         for (int x = 0; x < m_size; x++)
         {
             for (int y = 0; y < m_size; y++)
             {
                 float noiseValue = Noise.GeneratePerlin2D(x + noisePos.x, y + noisePos.y, m_frequency, m_octaves, m_amplitude, m_lacunarity, m_invLacunarity);
                 tex.SetPixel(x, y, new Color(noiseValue, noiseValue, noiseValue));
             }
         }
 
         tex.Apply();
         m_sharedQuadMat.mainTexture = tex;
 
         Mesh mesh = m_chunks[0].GetComponent<MeshFilter>().mesh;
         Vector3[] vertices = mesh.vertices; // Right here, mesh.vertices are all set to Vector3.zero.
         Vector3[] newVertices = new Vector3[vertices.Length];
 
         for (int i = 0; i < vertices.Length; i++)
         {
 
             Vector3 currentVertex = vertices[i];
             Vector3 worldPos = m_chunks[0].gameObject.transform.TransformPoint(vertices[i]);
             float noiseValue = Noise.GeneratePerlin2D(worldPos.x + noisePos.x, worldPos.y + noisePos.y, m_frequency, m_octaves, m_amplitude, m_lacunarity, m_invLacunarity);
             noiseValue *= 100;
 
             currentVertex = new Vector3(currentVertex.x, noiseValue, currentVertex.z);
             newVertices[i] = currentVertex;
 
             Debug.Log("Value: " + currentVertex);
         }
 
         mesh.vertices = newVertices;
 
         mesh.RecalculateNormals();
         mesh.RecalculateBounds();
         mesh.RecalculateTangents();
 
         //m_chunks[0].GetComponent<Material>().mainTexture = tex;
 
         //m_sharedQuadMat.SetTexture("_BumpMap", tex);
         //m_sharedQuadMat.SetTexture("_HeightMap", tex);
         //m_sharedQuadMat.SetTexture("_Occlusion", tex);
     }
 
 }
 
 #if UNITY_EDITOR
 
 [CustomEditor(typeof(WorldManager))]
 public class WorldManagerEditor : Editor
 {
     private Vector3 lastPos;
 
     public override void OnInspectorGUI()
     {
         base.OnInspectorGUI();
 
         if (GUILayout.Button("Generate Preview"))
             ((WorldManager)target).GeneratePreviewNoise();
     }
 }
 
 #endif