// Animate an elementary move
animateMove: function(move, callback) {
let startSquare = document.getElementById(getSquareId(move.start));
+ if (!startSquare) return; //shouldn't happen but...
let endSquare = document.getElementById(getSquareId(move.end));
let rectStart = startSquare.getBoundingClientRect();
let rectEnd = endSquare.getBoundingClientRect();