How to use RenderStaticPreview OR get project viewer thumbnails for ScriptableObject assets?

I'm using the code from the example here

Here is how my current script looks (Tile3D is a ScriptableObject, it's sub-classing another base class I wrote that doesn't have much going on it it yet.)

 using System.Collections;
 using System.Collections.Generic;
 using System.IO;
 using UnityEngine;
 using UnityEditor;
 
 [CustomEditor(typeof(Tile3D))]
 public class Tile3DEditor : Editor
 {
     public override Texture2D RenderStaticPreview(string assetPath, UnityEngine.Object[] subAssets, int width, int height)
     {

         Tile3D tile = target as Tile3D;
 
         if (tile == null)
             return null;
 
         Texture2D cache = new Texture2D(width, height);
         EditorUtility.CopySerialized(AssetPreview.GetAssetPreview(tile.mat.mainTexture), cache);
         
         return cache;
     }
 }
 
   
 public class Tile3D : CGridObject
 {
     public GameObject shape;
     public Material mat;


     private void Awake()
     {
         mat = (Material)Resources.Load("Tilemap3D/assets/DefaultMat");
         shape = (GameObject)Resources.Load("Tilemap3D/assets/TileCube");
     }
 
     [MenuItem("Assets/Create/Tilemap 3D/Tile3D")]
     public static void CreateTile3D()
     {
         string path = AssetDatabase.GetAssetPath(Selection.activeObject);
 
         if (path == "")
         {
             path = "/Tile3D/tile3D.Asset";
         }
         else
         {
             path += "/tile3D.Asset";
         }
 
         Tile3D asset = ScriptableObject.CreateInstance<Tile3D>();
 
         AssetDatabase.CreateAsset(asset, path);
     }
 }

It Almost works, but every time Unity starts up, the first time RenderStaticPreview() is called, AssetPreview.GetAssetPreview() returns null, and a NullReferenceException occurs. Every sequential call after that seems to be fine.

All I want is for my custom ScriptableObject assets to have their own custom icons in the project viewer, as well as the ability to keep them updated relative to a texture reference the object has. It seems like RenderStaticPreview() is the way to go about this, but what am I missing here? Is there a more proper way to go about using this method?

Or is there a more proper way to accomplish this task in general?