Ahh I see. This is indeed a strange problem. Though I don't think the object you said you added in the game could be the cause, as the same thing many people do in Unity as well. I dunno but this sure is a hella weird problem lol. I too faced it with my first game, though changing icon back to default worked for me.
But anyways, the game was really good, keep up the great work. And happy game developing!