Find Asset Path From Renderer Component

I am attempting to write an AssetPostprocessor script to make the type of material assigned to imported FBX files by default be "unlit/texture" instead of "Diffuse". It all works, except I cannot get AssetDatabase.GetAssetPath to return anything for me. It just returns an empty string.

I want to put the new material in the folder where the fbx was imported.

  using UnityEngine;
     using UnityEditor;
     
     class MaterialImport : AssetPostprocessor
     {
         Material OnAssignMaterialModel(Material material, Renderer renderer)
         {
             material.shader = Shader.Find( "Unlit/Texture" );
             string object_path = AssetDatabase.GetAssetPath(renderer.gameObject.GetInstanceID());
             AssetDatabase.CreateAsset(material, object_path + renderer.gameObject.name + ".mat");
             Debug.Log(object_path);
             return material;
         }
     }