Object reference not set to an instance of an object

I am trying to run this code for a game:

 function Start () {
   playerCanClick = true; // We should let the player play, don't you think?
 
   // Initialize some empty Collections:
   aCards = new List.<Card>(); // this Generic List is our deck of cards. It can only ever hold instances of the Card class.
   aGrid = new Card[rows,cols]; // The rows and cols variables help us define the dimensions of this 2D array
   aCardsFlipped = new List.<Card>(); // This List will store the two cards the player flips over.
   BuildDeck();
   // Loop through the total number of rows in our aGrid List:  
   for(var i:int = 0; i<rows; i++)
   {
   
       // For each individual grid row, loop through the total number of columns in the grid:
       for(var j:int = 0; j<cols; j++)
       {
         var someNum:int = Random.Range(0,aCards.Count);
          aGrid[i,j] = aCards[someNum];
          aCards.RemoveAt(someNum);
       }
     }
   }
   
   function OnGUI ()
   {
      GUILayout.BeginArea (Rect (0,0,Screen.width,Screen.height));
      BuildGrid();
      if(playerHasWon)BuildWinPrompt();
      GUILayout.EndArea();
   }
   
 function BuildGrid()
 {
   GUILayout.BeginVertical();
   GUILayout.FlexibleSpace();
   for(var i:int=0; i<rows; i++)
   {
     GUILayout.BeginHorizontal();
     GUILayout.FlexibleSpace();
     for(var j:int=0; j<cols; j++)
     {
       var card:Object = aGrid[i,j]; 
       var img:String;
       
       if(card.isMatched)
       {
            img = "blank";
       } else {
           if(card.isFaceUp)
             {
             img = card.img;
             }else{
             img = "faceup";
             }
         }    
           
       GUI.enabled = !card.isMatched;
       
       if(GUILayout.Button(Resources.Load(img), GUILayout.Width(80), GUILayout.Height(80)))
       {
           if(playerCanClick){
               FlipCardFaceUp(card);
       }
         Debug.Log(card.img);
       }
     }
     GUI.enabled = true;
     
     GUILayout.FlexibleSpace();
     GUILayout.EndHorizontal();   
   }
   GUILayout.FlexibleSpace();
   GUILayout.EndVertical();
 }
 
 function BuildDeck(){
     var card:Object; // this stores a reference to a card
     var id:int = 0;
     var num:int = 0;
        var otherNum:int = 2;
 
        var aColorParts:List.<String> = new List.<String>();
     aColorParts.Add("Green");
     aColorParts.Add("Yellow");
     aColorParts.Add("Red");
     aColorParts.Add("Orange");
     aColorParts.Add("Purple");
     aColorParts.Add("Pink");
     aColorParts.Add("LightBlue");
     aColorParts.Add("Blue");
    
     for(var j:int=0; j<2; j++)
     { 
            var theMissingPart:String = aColorParts[num];
    
            aColorParts.RemoveAt(num);
       
         card = new Card("color" + "Missing" + theMissingPart, id);
             aCards.Add(card);
          
         card = new Card("color" + theMissingPart, id);
         aCards.Add(card);
         
         if(otherNum % 2 == 1){
             num++;
         }
         otherNum++;
         id++;
    }
 }

When I run it, I get an error saying: "Object reference not set to an instance of an object" on this line:

 if(card.isMatched)

and an error saying: "Argument is out of range" on this line:

 aGrid[i,j] = aCards[someNum];

What is the problem?