I'm getting an error trying to add my class to org-mode

(add-to-list 'org-latex-classes ieeetran-class)
Symbol's value as variable is void: org-latex-classes

What is the org-mode 9 way of adding classes?

Also, on org-mode 8 i added this hook to remove headlines and it worked
perfectly, now it complains about 'tree' being a void variable too

(defun org-remove-headlines (backend)
  "Remove headlines with :no_title: tag."
  (org-map-entries (lambda () (let ((beg (point)))
                                (outline-next-visible-heading 1)
                                (backward-char)
                                (delete-region beg (point))))
                   "no_export" tree)
  (org-map-entries (lambda () (delete-region (point-at-bol) (point-at-eol)))
                   "no_title"))

(add-hook 'org-export-before-processing-hook #'org-remove-headlines)