Solved the error in the whatPiece

src/backend/Board.java

@@ -124,7 +124,7 @@ public class Board {
 	public int whatPiece(int x, int y) { // method which gives the index (in the pieces array) of the piece at a position x,y 
 		boolean pieceHere = false;
 		int index = 0;
-		while (pieceHere == false) {
+		while (pieceHere == false  && index != pieces.size()) {
 			if((x == pieces.get(index).getX()) && (y == pieces.get(index).getY())){
 				pieceHere =true;
 			}
@@ -132,6 +132,9 @@ public class Board {
 				index += 1;
 			}
 		}
+		if(index == pieces.size()) {
+			index=-1;
+		}
 		return index;
 	}