I can confirm this bug occurs also on my customized installers. I unfortunately still generate my installers from Debian 10.13 and 11.9 Netinst images (not enough time to migrate to Debian 12). When I recently switched to UEFI booting with my GRUB theme
without a background image I started to get the "error: null src bitmap in grub_video_bitmap_create_scaled." message.

My GRUB menu does not have submenus, but the error message appears after I press ENTER to run any GRUB menu entry. Booting of the given entry continues after a delay or when one hits a key, but it still makes the user experience feel "buggy".

My GRUB theme uses "desktop-color" instead of "desktop-image" which is probably the cause of the error message. I found a workaround here:
https://github.com/grml/grml-live/commit/8a09c98f94a43bdccd8699e200b66ab81f9102bf

But on my installed systems this same GRUB theme works without that error, which is odd as the GRUB versions are (nearly) the same for the installers and installed systems. Anyway, since I want to keep my GRUB theme nice and simple + same for installers
as well as installed systems, I do not want images in the theme. I use the theme just to be able to change the font, menu positioning and texts (title, messages). Thus, it would be nice if this bug gets fixed or if someone suggests a better workaround.

Regards,
Jmkr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)