corfu raise error on completion-at-point-functions function return nil

[stardiviner]

Reproduce:

1. setting completion-at-point-functions for python-mode to the value:

       (use-package cape
         :ensure t
         ;; [ yasnippet-cape ] -- A simple capf (Completion-At-Point Function) for completing yasnippet snippets.
         :ensure yasnippet-capf
         :config
         ;; Setting `completion-at-point-functions', used by `completion-at-point'.
         (setq-default completion-at-point-functions
                       (list
                        #'cape-file
                        (if (featurep 'yasnippet-capf)
                            #'yasnippet-capf
                          (cape-company-to-capf 'company-yasnippet))
                        #'cape-keyword
                        #'cape-sgml
                        #'cape-tex
                        #'cape-emoji
                        #'cape-abbrev
                        ;; #'cape-line
                        (if (featurep 'tabnine)
                            #'tabnine-completion-at-point
                          (when (featurep 'company-tabnine)
                            (cape-company-to-capf 'company-tabnine)))
                        )))
       
       (use-package lsp-mode
         :ensure t
         :ensure lsp-pyright
         :defer t
         :hook (python-mode . lsp)
         :config (require 'lsp-pyright) (require 'dap-python))
       
       (add-hook 'python-mode-hook
                 (lambda ()
                   (setq-local completion-at-point-functions
                               '(lsp-completion-at-point python-completion-at-point t))))

2. open an Org mode file contains a Python source block:

      #+begin_src python
      import sys|
      #+end_src
   

3. press [C-c '] to open the Python source block in a dedicated buffer.

   NOTE: The lsp is not enabled in the new dedicated buffer. So I guess because the lsp-completion-at-point in
   completion-at-point-functions return nil. Then it raise error by corfu:

    Debugger entered--Lisp error: (void-function nil)
      nil()
      corfu--capf-wrapper(nil)
      run-hook-wrapped(corfu--capf-wrapper nil)
      corfu--auto-complete-deferred((#<window 456 on My Python Language Syntax Reference.py> #<buffer My Python Language Syntax Reference.py> 60 117))
      apply(corfu--auto-complete-deferred (#<window 456 on My Python Language Syntax Reference.py> #<buffer My Python Language Syntax Reference.py> 60 117))
      timer-event-handler([t 26258 1015 531654 nil corfu--auto-complete-deferred ((#<window 456 on My Python Language Syntax Reference.py> #<buffer My Python Language Syntax Reference.py> 60 117)) nil 0 nil])

• Emacs version: GNU Emacs 30.0.60 (build 2, aarch64-apple-darwin23.5.0, NS appkit-2487.60 Version 14.5 (Build 23F79)) of 2024-06-30
• corfu version: Version: 20240710.1000
  Commit: c1d75d3

[minad]

Your completion-at-point-functions are not setup correctly. It seems the list of functions contains the value nil. The tabnine conditional is the problem.