Runtime generated mesh is not shaded right,only grey?

Hello to all,

I am trying to draw a cube at runtime but I am getting a result something like in the image. What would be the possible reasons for that? Thank you.

Edit: Ok I am giving some code too. This is how I create meshes.

     var l = new List<Vector3>();
     var bh = new BuildingHolder(center, l,buildingHeight);
     
     for (int i = 0; i < l.Count; i++)
     {
            l[i] = l[i] - bh.Center;
     }
     
     BuildingDictionary.Add(center, bh);        
     var m = bh.CreateModel();
     m.name = "building";
     m.transform.parent = this.transform;
     m.transform.localPosition = center;

BuildingHolder.cs

 public class BuildingHolder
     {
         public GameObject GameObject { get; set; }
         public Vector3 Center { get; set; }
         public float Rotation { get; set; }
         public bool IsModelCreated;
         private List<Vector3> _verts;
         private float buildingHeight;
 
         public BuildingHolder(Vector3 center, List<Vector3> verts, float height)
         {
             IsModelCreated = false;
             Center = center;
             _verts = verts;
             buildingHeight = height;
         }
 
         public GameObject CreateModel()
         {
             if (IsModelCreated)
                 return null;
 
             var m = new GameObject().AddComponent<BuildingPolygon>();
             m.gameObject.transform.position = Center;
             m.Initialize(_verts,buildingHeight);
             IsModelCreated = true;
             return m.gameObject;
         }
 
     }

BuildingPolygon.cs

 [RequireComponent(typeof(MeshFilter), typeof(MeshRenderer))]
     class BuildingPolygon : MonoBehaviour
     {
 
         private static float buildingHeight;
 
         public void Initialize(List<Vector3> verts, float height)
         {
             GetComponent<MeshFilter>().mesh = CreateMesh(verts);
             GetComponent<MeshRenderer>().material = Resources.Load("BuildingMat") as Material;
             buildingHeight = height;
         }
         private static Mesh CreateMesh(List<Vector3> verts)
         {
             var tris = new Triangulator(verts.Select(x => x.ToVector2xz()).ToArray());
             var mesh = new Mesh();
 
             var vertices = verts.Select(x => new Vector3(x.x, 0, x.z)).ToList();
             var indices = tris.Triangulate().ToList();
             //var uv = new List<Vector2>();
 
             var n = vertices.Count;
             for (int index = 0; index < n; index++)
             {
                 var v = vertices[index];
                 vertices.Add(new Vector3(v.x, buildingHeight, v.z));
             }
 
             for (int i = 0; i < n - 1; i++)
             {
                 indices.Add(i);
                 indices.Add(i + n);
                 indices.Add(i + n + 1);
                 indices.Add(i);
                 indices.Add(i + n + 1);
                 indices.Add(i + 1);
             }
 
             indices.Add(n - 1);
             indices.Add(n);
             indices.Add(0);
 
             indices.Add(n - 1);
             indices.Add(n + n - 1);
             indices.Add(n);  
 
             mesh.vertices = vertices.ToArray();
             mesh.triangles = indices.ToArray();
 
             mesh.RecalculateNormals();
             mesh.RecalculateBounds();
 
             return mesh;
         }
     }