Unable to read memory - returning abstract class pointer from another class

I have an abstract class (pure virtual function in it) and I use polymorphism so it'd contain an address of a class inheriting of that class. The 'parent' is called Piece and the class containing a pointer to it is Square. Anyways, I'm getting a weird exception, with the following code:

Header files:

    #pragma once //Square.h
class Piece; //as I cannot include piece.h, if i do, I actually include myself, because piece.h includes board.h and board.h includes me.
class Square
{
public:
    Square(void);
    Piece* getPiece();
    void setPiece(Piece* piece);
    ~Square(void);
private:
    Piece* _piece;
};

And a second one:

#pragma once
#include "Square.h"
class Board
{
public:
    Board(void);
    Square& getSquare(int row, int col);
    ~Board(void);
private:
    Square** _square;
};

And a third for last:

#pragma once
#include <iostream>
#include "Coord.h"
#include "Board.h"
using namespace std;

class Piece //irellevant variables though, for this question
{
public:
    Piece(Coord pos, bool color);
    virtual bool isLegalMove(Coord c, Board* b) const = 0;
    virtual char getPieceChar() const = 0;
    ~Piece(void);
protected:
    Coord _pos;
    bool _color;
};

and the code itself wherever I crash upon is Square's getPiece() function (when I call it from the main):

Piece* Square::getPiece()
{
    return this->_piece;
}

I know that in that square, the _piece Piece pointer is null, and yet the program crashes here. I'm on windows with VS 2012 Ultimate and here is the Exception message:

Access violation reading location 0x000000C4.

and it says about this->_piece (when I look at the bottom variables), 'unable to read memory', but that's to expect as the pointer is NULL.

Any idea what's going on, making my program crash ?