var Cell = Container.expand(function (type) {
	var self = Container.call(this);
	var cellGraphics = self.createAsset('cell', 'Grid Cell', .5, .5);
	self.targetX = 0;
	self.targetY = 0;
	self.isMoving = false;
	self.totalTypes = 5;
	self.type = type || Math.floor(Math.random() * self.totalTypes);
	var hue = self.type / self.totalTypes;
	var color = hsvToRgb(hue, 1, 1);
	cellGraphics.tint = color;
	self.move = function (x, y, instant) {
		self.targetX = x;
		self.targetY = y;
		if (instant) {
			self.x = x;
			self.y = y;
		}
	};
	self.tick = function () {
		var acceleration = 1;
		self.speedX = self.speedX || 0;
		self.speedY = self.speedY || 0;
		var threshold = 1;
		var dx = self.targetX - self.x;
		var dy = self.targetY - self.y;
		if (Math.abs(dx) < threshold && Math.abs(dy) < threshold) {
			self.x = self.targetX;
			self.y = self.targetY;
			self.isMoving = false;
			self.speedX = 0;
			self.speedY = 0;
		} else {
			if (dx !== 0) {
				self.speedX += dx > 0 ? acceleration : -acceleration;
			}
			if (dy !== 0) {
				self.speedY += dy > 0 ? acceleration : -acceleration;
			}
			var nextX = self.x + self.speedX;
			var nextY = self.y + self.speedY;
			if (self.speedX > 0 && nextX > self.targetX || self.speedX < 0 && nextX < self.targetX) {
				nextX = self.targetX;
				self.speedX = 0;
			}
			if (self.speedY > 0 && nextY > self.targetY || self.speedY < 0 && nextY < self.targetY) {
				nextY = self.targetY;
				self.speedY = 0;
			}
			self.x = nextX;
			self.y = nextY;
			self.isMoving = self.x !== self.targetX || self.y !== self.targetY;
		}
	};
});
function hsvToRgb(h, s, v) {
	var i = Math.floor(h * 6), f = h * 6 - i, p = v * (1 - s), q = v * (1 - f * s), t = v * (1 - (1 - f) * s), mod = i % 6, r = [v, q, p, p, t, v][mod], g = [t, v, v, q, p, p][mod], b = [p, p, t, v, v, q][mod];
	return (r * 255 << 16) + (g * 255 << 8) + b * 255;
}
var Game = Container.expand(function () {
	var self = Container.call(this);
	var gridWidth = 4;
	var gridHeight = 4;
	var gridSpacing = 8;
	var tempCell = new Cell();
	var cellWidth = tempCell.width;
	var cellHeight = tempCell.height;
	tempCell.destroy();
	var gridContainer = new Container();
	self.addChild(gridContainer);
	var totalGridWidth = gridWidth * (cellWidth + gridSpacing) - gridSpacing;
	var totalGridHeight = gridHeight * (cellHeight + gridSpacing) - gridSpacing;
	gridContainer.x = (2048 - totalGridWidth) / 2 + cellWidth / 2;
	gridContainer.y = (2732 - totalGridHeight) / 2 + cellHeight / 2;
	self.calculateTargetPosition = function (col, row) {
		return {
			x: col * (cellWidth + gridSpacing),
			y: row * (cellHeight + gridSpacing)
		};
	};
	self.findConnectedNeighbors = function (cell, connectedNeighbors) {
		connectedNeighbors = connectedNeighbors || [];
		if (!cell) return [];
		var cellType = cell.type;
		var cellColRow = self.findCellColRow(cell);
		if (cellColRow) {
			var directions = [[-1, 0], [1, 0], [0, -1], [0, 1]];
			directions.forEach(function (dir) {
				var newRow = cellColRow.row + dir[0];
				var newCol = cellColRow.col + dir[1];
				if (newRow >= 0 && newRow < gridHeight && newCol >= 0 && newCol < gridWidth) {
					var neighborCell = grid[newCol][newRow];
					if (neighborCell && neighborCell.type === cellType && connectedNeighbors.indexOf(neighborCell) === -1) {
						connectedNeighbors.push(neighborCell);
						self.findConnectedNeighbors(neighborCell, connectedNeighbors);
					}
				}
			});
		}
		return connectedNeighbors;
	};
	self.findCellColRow = function (cell) {
		for (var col = 0; col < gridWidth; col++) {
			for (var row = 0; row < gridHeight; row++) {
				if (grid[col][row] === cell) {
					return {
						col: col,
						row: row
					};
				}
			}
		}
		return null;
	};
	self.deleteCell = function (cell) {
		var colRow = self.findCellColRow(cell);
		if (colRow) {
			grid[colRow.col][colRow.row] = null;
			cell.destroy();
		}
	};
	self.attachCellListeners = function (cell) {
		cell.on('down', function (obj) {
			var cellsToDestroy = [cell].concat(self.findConnectedNeighbors(cell));
			cellsToDestroy.forEach(function (cell) {
				self.deleteCell(cell);
			});
			self.moveToEmptySpace('down', false);
			self.moveToEmptySpace('left', true);
		});
		cell.on('up', function (obj) {});
		cell.on('move', function (obj) {});
	};
	var grid = [];
	for (var i = 0; i < gridWidth; i++) {
		grid[i] = [];
		for (var j = 0; j < gridHeight; j++) {
			var cell = new Cell();
			var targetPos = self.calculateTargetPosition(i, j);
			cell.move(targetPos.x, targetPos.y, true);
			self.attachCellListeners(cell);
			grid[i][j] = cell;
			gridContainer.addChild(cell);
		}
	}
	self.moveToEmptySpace = function () {
		var moved;
		do {
			moved = false;
			for (var col = 0; col < gridWidth; col++) {
				for (var row = gridHeight - 2; row >= 0; row--) {
					if (grid[col][row] && !grid[col][row + 1]) {
						var targetCell = grid[col][row];
						var targetPos = self.calculateTargetPosition(col, row + 1);
						targetCell.move(targetPos.x, targetPos.y);
						grid[col][row + 1] = targetCell;
						grid[col][row] = null;
						moved = true;
					}
				}
			}
		} while (moved);
		for (var col = 0; col < gridWidth; col++) {
			if (grid[col] && !grid[col][0]) {
				var newCell = new Cell();
				var targetPos = self.calculateTargetPosition(col, 0);
				newCell.x = targetPos.x;
				newCell.y = targetPos.y - cellHeight - 200;
				LK.setTimeout(function () {
					newCell.move(targetPos.x, targetPos.y);
					self.attachCellListeners(newCell);
					grid[col][0] = newCell;
					gridContainer.addChild(newCell);
				}, 500);
			}
		}
	};
	LK.on('tick', function () {
		for (var i = 0; i < gridWidth; i++) {
			for (var j = 0; j < gridHeight; j++) {
				if (grid[i][j]) grid[i][j].tick();
			}
		}
	});
});

===================================================================
--- original.js
+++ change.js
@@ -160,9 +160,9 @@
 				}
 			}
 		} while (moved);
 		for (var col = 0; col < gridWidth; col++) {
-			if (!grid[col][0]) {
+			if (grid[col] && !grid[col][0]) {
 				var newCell = new Cell();
 				var targetPos = self.calculateTargetPosition(col, 0);
 				newCell.x = targetPos.x;
 				newCell.y = targetPos.y - cellHeight - 200;