Unable to quit emacs with `Wrong type argument: hash-table-p, nil`

Question

I have some customisations in ~/.emacs.el. Today I refactored a piece that used dolist, directory-files, and find-file. I am now unable to quit Emacs in the normal way and I get this from the mini-buffer:

Wrong type argument: hash-table-p, nil

I only have one kill hook, which calls a function mu4e-mark-execute-all defined in mu4e:

(require 'mu4e)
(defun mu4e-execute-marks ()
   (interactive)
   (mu4e-mark-execute-all t)
)
(add-hook 'kill-emacs-hook 'mu4e-execute-marks)

I understand that the LISP debugger is for evaluating expressions or functions. Can I set a stacktrace to see the cause of the error?

Answer 1

What is the actual problem and what can I do about it?

The hash is only initialized in mu4e-headers-mode. Therefore, you should correct mu4e-execute-marks in the following way if you want to call it in kill-emacs-hook:

(defun mu4e-execute-marks ()
   (interactive)
   (when (derived-mode-p 'mu4e-headers-mode)
     (mu4e-mark-execute-all t))
)

How to debug such errors?

Switch on "Enter Debugger on Error" in the "Options"-menu. That meu item runs the command toggle-debug-on-error that just toggles the value of the variable debug-on-error between nil and t.

First sentence of the doc of debug-on-error:

Non-nil means enter debugger if an error is signaled.

The debugger includes the backtrace.

Alternatively you can instrument mu4e-mark-execute-all with M-x edebug-on-entry RET mu4e-mark-execute-all RET.

edebug is an interactive debugger. You can step through the function with the space button and you can display the backtrace with the menu item "Edebug"->"Views"->"Show Backtrace".