Sorry, but you really aren't really reading nor the code nor the comments, plus you're not really helping, the error only occurs in my engine, that's why I'm not posting it as a bug question, but rather as a question only.
if you read the game class there's a method there with a comment saying exactly what it does, that I call in initialize to pass the window object to the drawingManager object in the initialize method, you're just not paying any attention, I commented on the side of each method in the drawing manager saying EXACTLY what it does in order not to make the post even longer.