Swap GameObject before instantiating inside for each loop

Hi There! I have a bit of problem. I'm deveopling a city building game, and i wrote a simple save and load system, based on txt files. However, while im loading my save debugger shows me that i'm trying to instantiate null(block not assigned) or array index is our of range. Block selection works like this: i got three variables block - which is the "selector", block_a1 - 1st type of building, and block_a2 - 2nd type of building. Depending on what save file says loading the save instantiates block prefab, which becomes block_a1 or block_a2. So that's how it works in theory - now, here's the code: #pragma strict

 import System;
 import System.IO;
 
 var strArr : String[] = new String[100];
 var splitLine : String[] = new String[100];
 var fileName : String = "save.txt";
 var line : String[];
 var buildingType : String;
 //selected block
 var block : GameObject;
 //types of buildings
 var block_a1 : GameObject;
 var block_a2 : GameObject;
 
 
  function ReadData () {
  //reader
      var sr = new StreamReader(fileName);
      var fileContents = sr.ReadToEnd();
      sr.Close();
  //split txt into lines
      line = fileContents.Split("\n"[0]);
      for (var i = 0; 0 < line.length; i++) {
      strArr = line[i].Split("|"[0]);
 //parse vector3 from txt   
      splitLine = strArr[0].Split(" "[0]);
      var position : Vector3;
      position.x = parseFloat(splitLine[0]);
      position.y = parseFloat(splitLine[1]);
      position.z = parseFloat(splitLine[2]);
 //check for type     
      if(buildingType == "a1"){
      block = block_a1;
      }
      if(buildingType == "a2"){
      block = block_a2;
      }
 //instantiate     
      buildingType = strArr[1];
      var building : GameObject;
      building = Instantiate(block, position, Quaternion.identity);
      building.name = buildingType;
   }
 }
 
 
 function Update () {
 
 if(Input.GetKeyDown(KeyCode.L)){
 Invoke("ReadData",0.5);
 }
 
 }

Example save file looks like this:

 x y z|a1
 x y z|a2

x, y and z are position coordinates, and a1 or a2 is a block type. Objects are stored one per file line.

My question - what am I doing wrong? Thanks in advance.