Object2Terrain Creates a Invalid Collider. Help please.

Hey there. I was having the same issue as you today and stumbled upon your post. My Terrain object had its data somewhere, but the actual asset didn't exist so the Terrain Collider wouldn't work properly. I know it's two years late, but here's a little workaround.

If you open up Object2Terrain.cs, you can see it create the Terrain Data object, but it never gets added as an asset. You can do it yourself with the function call AssetDatabase.CreateAsset (terrain, path). Just make sure the path is relative to your project. Then you just use the tool again and the terrain data will show up in your assets. You can drag that into the terrain collider data field. My new CreateTerrain function looks like this:

     void CreateTerrain(){    
  
         //fire up the progress bar
         ShowProgressBar(1, 100);
  
         TerrainData terrain = new TerrainData();
         terrain.heightmapResolution = resolution;
         GameObject terrainObject = Terrain.CreateTerrainGameObject(terrain);
  
         Undo.RegisterCreatedObjectUndo(terrainObject, "Object to Terrain");
  
         MeshCollider collider = Selection.activeGameObject.GetComponent<MeshCollider>();
         CleanUp cleanUp = null;
  
         //Add a collider to our source object if it does not exist.
         //Otherwise raycasting doesn't work.
         if(!collider){
  
             collider = Selection.activeGameObject.AddComponent<MeshCollider>();
             cleanUp = () => DestroyImmediate(collider);
         }
  
         Bounds bounds = collider.bounds;    
         float sizeFactor = collider.bounds.size.y / (collider.bounds.size.y + addTerrain.y);
         terrain.size = collider.bounds.size + addTerrain;
         bounds.size = new Vector3(terrain.size.x, collider.bounds.size.y, terrain.size.z);
  
         // Do raycasting samples over the object to see what terrain heights should be
         float[,] heights = new float[terrain.heightmapWidth, terrain.heightmapHeight];    
         Ray ray = new Ray(new Vector3(bounds.min.x, bounds.max.y + bounds.size.y, bounds.min.z), -Vector3.up);
         RaycastHit hit = new RaycastHit();
         float meshHeightInverse = 1 / bounds.size.y;
         Vector3 rayOrigin = ray.origin;
  
         int maxHeight = heights.GetLength(0);
         int maxLength = heights.GetLength(1);
  
         Vector2 stepXZ = new Vector2(bounds.size.x / maxLength, bounds.size.z / maxHeight);
  
         for(int zCount = 0; zCount < maxHeight; zCount++){
  
             ShowProgressBar(zCount, maxHeight);
  
             for(int xCount = 0; xCount < maxLength; xCount++){
  
                 float height = 0.0f;
  
                 if(collider.Raycast(ray, out hit, bounds.size.y * 3)){
  
                     height = (hit.point.y - bounds.min.y) * meshHeightInverse;
                     height += shiftHeight;
  
                     //bottom up
                     if(bottomTopRadioSelected == 0){
  
                         height *= sizeFactor;
                     }
  
                     //clamp
                     if(height < 0){
  
                         height = 0;
                     }
                 }
  
                 heights[zCount, xCount] = height;
                    rayOrigin.x += stepXZ[0];
                    ray.origin = rayOrigin;
             }
  
             rayOrigin.z += stepXZ[1];
               rayOrigin.x = bounds.min.x;
               ray.origin = rayOrigin;
         }
  
         terrain.SetHeights(0, 0, heights);

             // Create asset file here
         AssetDatabase.CreateAsset (terrain, "Assets/terrainData.asset");
 
         EditorUtility.ClearProgressBar();
  
         if(cleanUp != null){
  
             cleanUp();    
         }
     }