Add Trees on Random Generated Terrain

I have used a script to make from a game object a random terrain and now I am trying to add random trees and then convert the gameObject to a Terrain

 public GameObject terrainObject;
 Terrain terrain = terrainObject.GetComponent<Terrain>();

But when I try to add trees on it with this code it doesn't work :

 .............................
 TreeInstance t = new TreeInstance();
 t.color = Color.white;
 t.heightScale = 1;
 t.widthScale = 1;
 t.lightmapColor = Color.white;
 t.position = new Vector3(x, 0, z);
 terrain.AddTreeInstance(t);

And I also get the following error when I try to do anything with the terrain gameobject

 Object reference not set to an instance of an object

It seems that it doesn't transform the game object to a terrain.