Variable ac-trigger-key has default value nil which does not match its
customize type

    :type 'string

This causes M-x customize-variable ac-trigger-key to show "mismatch" and
edit as a lisp form, which is rather tricky for a beginner (or for that
matter if you don't realize that's what happens for "mismatch").

Some other variables taking nil or string use a choice type like

    :type '(choice (const :tag "None" nil) string)

An alternative could be empty string to mean no trigger key.  That might
happen to work already due to define-key doing nothing on an empty key
string.  Dunno if it's a good idea to rely on that from define-key.
(ac-set-trigger-key could always check for empty string itself if it
seemed like a good idea.)