Number 2 may be correct, but the meaning is different. "if" in this case means "whether", so in the second sentence we ask not about opinion (sentence number one), but about the "activity of thinking" itself. It is the rare case, when "think" can come with -ing ending.I think the film is awesome
. vs I'm thinking about going to the cinema.
So, in theory both sentences are grammaticaly correct, but the second one is highly unusual.