Null pointer exception when playing with AI

[Longwater1234]

@Mchristos When playing against AI, after the AI did a double jump, i got NPE in the console as follows. it seems it is coming from line 192 of BoardState. I had not modified the original code.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "org.davistiba.game.Piece.getPlayer()" because the return value of "org.davistiba.game.BoardState.getPiece(int)" is null
	at org.davistiba.game.BoardState.getSuccessors(BoardState.java:192)
	at org.davistiba.game.BoardState.getSuccessors(BoardState.java:170)
	at org.davistiba.game.Game.getValidMoves(Game.java:79)