I'm working on a school project, I have to build a tic tac toe game with the AI based on the MiniMax algorithm. The two player mode works like it should.

I followed the code example on http://ethangunderson.com/blog/minimax-algorithm-in-c/. The only thing is that I get a NullPointer Exception when I run the code. And I can't wrap my finger around it.

I placed a comment in the code where the exception is thrown. The recursive call is returning a null pointer, what is very strange because it can't.. When I place a breakpoint on the null return with the help of a if statement, then I see that there ARE still 2 to 3 empty places.. I probably overlooking something.

Hope someone can tell me what I'm doing wrong.

Here is the MiniMax code (the tic tac toe code is not important):

/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/

package MiniMax;

import Game.Block;
import Game.Board;
import java.util.ArrayList;


public class MiniMax {
public static Place getBestMove(Board gameBoard, Block.TYPE player) {
    Place bestPlace = null;
    ArrayList<Place> emptyPlaces = gameBoard.getEmptyPlaces();
    Board newBoard;

    //loop trough all the empty places
    for(Place emptyPlace : emptyPlaces) {
        newBoard = gameBoard.clone();
        newBoard.setBlock(emptyPlace.getRow(), emptyPlace.getCell(), player);

        //no game won and still room to move
        if(newBoard.getWinner() == Block.TYPE.NONE && newBoard.getEmptyPlaces().size() > 0) {                
            //is an node (has children)
            Place tempPlace = getBestMove(newBoard, invertPlayer(player));

            //ERROR is thrown here! tempPlace is null.
            emptyPlace.setScore(tempPlace.getScore());
        } else {
            //is an leaf
            if(newBoard.getWinner() == Block.TYPE.NONE) {
                emptyPlace.setScore(0);
            } else if(newBoard.getWinner() == Block.TYPE.X) {
                emptyPlace.setScore(-1);
            } else if(newBoard.getWinner() == Block.TYPE.O) {
                emptyPlace.setScore(1);
            }

            //if this move is better then our prev move, take it!
            if((bestPlace == null)
                    || (player == Block.TYPE.X && emptyPlace.getScore() < bestPlace.getScore())
                    || (player == Block.TYPE.O && emptyPlace.getScore() > bestPlace.getScore())) {
                bestPlace = emptyPlace;
            }
        }
    }

    //This should never be null, but it does..
    return bestPlace;
}

private static Block.TYPE invertPlayer(Block.TYPE player) {
    if(player == Block.TYPE.X) {
        return Block.TYPE.O;
    }

    return Block.TYPE.X;
}
}