What was Joan Rivers holocaust joke?

In May 2014, Rivers made a controversial joke while presenting an award at the Television Critics Association Awards, stating, "I've been really busy writing. I have a new book coming out. It's called 'Diary of a Mad Fashionista'. It's about going into, mostly the department stores, and shoplifting. I would also like to thank the Holocaust. Without them, I wouldn't have half of my wardrobe."