TETRIS STYLE - create a preview of the next block

Hello I am trying to add some code or a function that would show a preview of the next block that is about to drop in a game of tetris (foundation for the game taken from Noobtuts). I have created a GameObject that would be considered the space where the preview would be shown, but whenever I try different methods it literally crashes Unity everytime. A snippet of code that I was trying to do was have the block that is about to spawn and the block that is next pulling from the same array, as I’m not sure how I could go about setting bools to pull from one and then convert it to the block dropped.

From Group.cs:

		else if (Input.GetKey (KeyCode.DownArrow) && !Pause.paused || Time.time * LevelUpdater.fallTimer - lastFall >= 1)
			//Modify position
			transform.position += new Vector3(0, -1, 0);

			//See if valid
			if (isValidGridPos())
				//It's valid.  Update grid.

				//It's not valid.  Revert.
				transform.position += new Vector3(0, 1, 0);

				//Clear filled horizontal lines

				//Spawn next Group
				previewDropped = true;

				//Disable script
				enabled = false;

			lastFall = Time.time;

Then this is the Spawner.cs:

public void spawnNext()
		//Random Index
		int i = Random.Range (0, groups.Length);

		if (!Group.previewDropped) 
			//Spawn Group at Preview Spawn Window Position
			Instantiate (groups *, previewSpawnWindow.transform.position, Quaternion.identity);*
  •  }*
  •  if (Group.previewDropped)* 
  •  {*

_ Instantiate (groups , transform.position, Quaternion.identity);_
* Group.previewDropped = false;*
* }*
* }*

Try this for C#

I can also send you my old scripts if that would any kind of help. Long long time ago I managed to get that: