Error during redisplay with c-mode submode

[matteo-l]

When editing buffers that contain snippets of c code the font locking is often not correct and in the Messages buffer repeatedly appears the following message:

Error during redisplay: (jit-lock-function 1) signaled (wrong-type-argument stringp nil)

The number after jit-lock-function is different for every error.

For example you can define the following class and apply it to a pio file (e.g. hello.pio from official examples

(mmm-add-classes
 ;; C code in asm, like in pio files for raspberry pi pico
 '((asm-c
    :submode c-mode
    :front "% c-sdk {\n"
    :back "%}")))

My configuration is

(require 'mmm-auto)
(setopt mmm-global-mode 'maybe)

followed by the class definition.

Emacs 30.1 built from source on Ubuntu 20.04 LTS