"ArgumentException: The Object you want to instaintiate is null."

Hi everyone,

Recently I'm getting this ArgumentException while attempting to instantiate a Terrain.
I first click on a button, then the Run() void (code below) is executed.
The exception is thrown at line 33.
Code for LevelLoader.cs:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class LevelLoader : MonoBehaviour {
     public class LevelElemTag
     {
         public Dictionary<string, string> TagProperties = new Dictionary<string, string>();
     }
 
     public string fileName = "";
     public void Run() {
         if (!System.IO.File.Exists(fileName)) { throw new System.Exception("File " + fileName + " not found!"); };
         //Load file content:
         string[] fileContent = System.IO.File.ReadAllLines(fileName, System.Text.Encoding.Unicode);
         //Process file:
         foreach(string s in fileContent) //For each element (represented as a text line in the file)
         {
             //Read base properties:
             Vector3 position = GetVector3(s,1);
             Quaternion rotation = GetQuaternion(s, 2);
             LevelElemTag data = GetTag(s, 3);
             //What is -s-?
             switch (GetNature(s))
             {
                 case "tran":
                     //It's a train model
                     Instantiate(GetTrain(data), position, rotation);
                     break;
                 case "terr":
                     //It's a terrain
                     Instantiate(GetTerrain(data), position, rotation);
                     break;
                 case "rail":
                     //It's a rail
                     Instantiate(GetRailway(data), position, rotation);
                     break;
                 case "deco":
                     //It's a decoration
                     Instantiate(GetDecor(data), position, rotation);
                     break;
                 case "virt":
                     //It's an entity (controller, effect...)
                     Instantiate(GetEntity(data), position, rotation);
                     break;
                 default:
                     break;
             }
         }
 
     }
     string GetNature(string line)
     {
         return line.Split('|')[0];
     }
     Vector3 GetVector3(string line,int index)
     {
         string block = line.Split('|')[index];
         float x = 0, y = 0, z = 0;
         try
         {
         x = float.Parse(block.Split(';')[0]);
         y = float.Parse(block.Split(';')[1]);
         z = float.Parse(block.Split(';')[2]);
         }
         catch (System.Exception e)
         {
             throw e;
         }
         return new Vector3(x, y, z);
     }
     Quaternion GetQuaternion(string line, int index)
     {
         string block = line.Split('|')[index];
         float w = 0,x = 0, y = 0, z = 0;
         try
         {
             w = float.Parse(block.Split(';')[0]);
             x = float.Parse(block.Split(';')[1]);
             y = float.Parse(block.Split(';')[2]);
             z = float.Parse(block.Split(';')[3]);
         }
         catch (System.Exception e)
         {
             throw e;
         }
         return new Quaternion(x,y,z,w);
     }
     LevelElemTag GetTag(string line, int index)
     {
         string block = line.Split('|')[index];
         LevelElemTag my = new LevelElemTag();
         foreach (string item in block.Split(';'))
         {
             my.TagProperties.Add(item.Split('=')[0], item.Split('=')[1]);
         }
         return my;
     }
 
     GameObject GetTrain(LevelElemTag meta)
     {
         GameObject model = (GameObject) Resources.Load("Trains/" + meta.TagProperties["family"] + "/" + meta.TagProperties["type"]) as GameObject;
         return model;
     }
     Terrain GetTerrain(LevelElemTag meta)
     {
         Terrain model = (Terrain)Resources.Load("Objects/Terrains/" + meta.TagProperties["family"] + "/" + meta.TagProperties["type"]) as Terrain;
         return model;
     }
     GameObject GetRailway(LevelElemTag meta)
     {
         GameObject model = (GameObject)Resources.Load("Objects/Rails/" + meta.TagProperties["family"] + "/" + meta.TagProperties["type"]) as GameObject;
         return model;
     }
     GameObject GetDecor(LevelElemTag meta)
     {
         GameObject model = (GameObject)Resources.Load("Objects/Decor/" + meta.TagProperties["family"] + "/" + meta.TagProperties["type"]) as GameObject;
         return model;
     }
     GameObject GetEntity(LevelElemTag meta)
     {
         GameObject model = (GameObject)Resources.Load("Objects/Entities/" + meta.TagProperties["family"] + "/" + meta.TagProperties["type"]) as GameObject;
         return model;
     }
 }
 

Directory structure for the terrain:
Assets/Objects/Terrains/debug/grassy.prefab (as in the editor's Project window)

Values at runtime:
family is debug
type is grassy

Thanks in advance
Lorenzo