Create a TreePrototype causes a object null exception

Hi, well I'm creating a procedural world generation, and well I like to add some trees to my terrains (is in plurar because I'm using a terrain tile system)...

Well, the Docs and community hasn't enough information for me, so, I started to code some hours ago, and I can't resolve this problem...

My problem is that when I want to assing the Tree Prototype (this window) to my TerrainData object:

Using code, instead of using the window before, a null reference exception stops all the process... I don't know exactly why, because Docs doesn't say anything or how to assing TreePrototype to a TerrainData with code...

Anyway, there is the code that you can check and tell me what I'm doing wrong...

This code is for generate the terrainData:

 public GameObject[] trees; //I used that because TreeProtype[] doesn't appear in Inspector...

 TerrainData GenerateTile() {

     TerrainData tData = new TerrainData();
     //... Properties
     tData.treePrototypes = GenerateTreeFromGOs(trees);
     //... More properties
     
     return tData;

 }

Well the function GenerateTreeFromGOs is that (and this is where I have the problem):

 TreePrototype[] GenerateTreeFromGOs(GameObject[] gos) {
     TreePrototype[] trees = new TreePrototype[gos.Length];
     for(int i  = 0; i < trees.Length; i++) {
         trees[i].bendFactor = m_bendFactor; //This part throw a null object exception
         trees[i].prefab = gos[i]; //And this too...
     }
     return trees;
 }

In this function I loop every assigned gameobject (via Inspector) and I add it to the prototypes...

So, how can I set the terrain Trees dynamically? Because, nobody (I think) has asked this before...

Thanks in advance.

Bye.