I very recently updated to Emacs 26.3 and Org 9.2.6, and am getting the above-mentioned error whenever I type "[[" in an org-mode buffer to type a hyperlink. Unsurprisingly, no error after turning off auto-complete mode (20170125.245, the only version I see in "list-packages").
Any possible fixes or recommendations?
Addendum 1: Aside from the error message, the link still works fine.
Addendum 2: Searching through the variable index of the Org manual, there's no mention of "org-link-types". The second line of the debugger backtrace cites "org-ac--get-link-head-candidates()". The hyperlink takes me to the function's definition in "org-ac.el" where there's a line
(mapcar (lambda (x) (concat x ":")) org-link-types))). Is it reasonable to assume this is what's causing the problem? And if so, is there anything to be done about it?