can't set terrain variable

Hello,

I found this script that generates grass onto the terrain, but I can't set my terrain on the according variable. Any help?

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 using System.IO;
 
 public class GrassCreator : ScriptableWizard  {
 
     public Terrain terrain;  //The variable that I can not assign the terrain
     public int detailIndexToMassPlace;
     public int[] splatTextureIndicesToAffect;
     public int detailCountPerDetailPixel = 0;
         
     [MenuItem ("Terrain/Mass Grass Placement")]
     
     static void createWizard() {
         
             ScriptableWizard.DisplayWizard("Select terrain to put grass on", typeof (GrassCreator), "Place Grass on Terrain");
         
     }
     
     void OnWizardCreate () {
         
         if (!terrain) {
             Debug.Log("You have not selected a terrain object");
             return;
         }
         
         if (detailIndexToMassPlace >= terrain.terrainData.detailPrototypes.Length) {
             Debug.Log("You have chosen a detail index which is higher than the number of detail prototypes in your detail libary. Indices starts at 0");
             return;
         }
         
         if (splatTextureIndicesToAffect.Length > terrain.terrainData.splatPrototypes.Length) {
             Debug.Log("You have selected more splat textures to paint on, than there are in your libary.");
             return;
         }
         
         for (int i = 0; i < splatTextureIndicesToAffect.Length; i ++) {
             if (splatTextureIndicesToAffect[i] >= terrain.terrainData.splatPrototypes.Length) {
                 Debug.Log("You have chosen a splat texture index which is higher than the number of splat prototypes in your splat libary. Indices starts at 0");
                 return;
             }
         }
         
         if (detailCountPerDetailPixel > 16) {
             Debug.Log("You have selected a non supported amount of details per detail pixel. Range is 0 to 16");
             return;
         }
         
         int alphamapWidth = terrain.terrainData.alphamapWidth;
         int alphamapHeight = terrain.terrainData.alphamapHeight;
         int detailWidth = terrain.terrainData.detailResolution;
         int detailHeight = detailWidth;
         
         float resolutionDiffFactor = (float)alphamapWidth/detailWidth;
         
         
         float[,,] splatmap = terrain.terrainData.GetAlphamaps(0,0,alphamapWidth,alphamapHeight);
         
         
         int[,] newDetailLayer = new int[detailWidth,detailHeight];
         
         //loop through splatTextures
         for (int i = 0; i < splatTextureIndicesToAffect.Length; i++) {
             
             //find where the texture is present
             for (int j = 0; j < detailWidth; j++) {
             
                 for (int k = 0; k < detailHeight; k++) {
                         
                     float alphaValue = splatmap[(int)(resolutionDiffFactor*j),(int)(resolutionDiffFactor*k),splatTextureIndicesToAffect[i]];
                         
                     newDetailLayer[j,k] = (int)Mathf.Round(alphaValue * ((float)detailCountPerDetailPixel)) + newDetailLayer[j,k];
             
                 }
                 
             }
             
         }
         
         terrain.terrainData.SetDetailLayer(0,0,detailIndexToMassPlace,newDetailLayer);    
         
     }
     
     void OnWizardUpdate ()
     {
         helpString = "Ready";
         
     
         
         
     }
     
 }