I finished the 1st draft of my book, and have put it in the freezer(as I asked about in this question). I was planning to start work on a sequel of the book in the meantime.
But I was told by someone that I should not work on a sequel till I have sold my first book, since if I can't sell the first book, I will never be able to sell the sequel either, and all my effort will be wasted.
Is this good advice? As it means all the effort I put in creating the characters, themes, Universe(it was a Sci-Fi/Fantasy book) will be wasted.