Does this happen with debug => 0 too?

You mean this setting? Some screenshots might help.

This is exactly what this option is about, but for the imported file.

The error appears in debug mode.

@ollehar this is the same area I'm working on for 19086. Do you want to give this one to me and I can fix them together?

also 19085

Ok I can reproduce on latest 5.x, PHP 8.0. Minimal steps from clean install:

1. Set debug > 0
2. Navigate Configuration -> Themes -> fruity -> Theme options -> Images tab
3. Settings: Background Image = Yes, Background Image File = Any
4. Click Save, you get the warning.

I would definitely change this from block to minor. It only happens with debug > 0 and its only a php warning, not an error. Also it doesn't break user workflow at all. Definitely fixable but not urgent.

When i do the 5.x PRs for the related issues, I have to touch that line of code anyways. So I'll address it then. Sorry @ollehar, I'm stealing it :P

Also reproduced on 6.x (latest master)

5.x: https://github.com/LimeSurvey/LimeSurvey/pull/3559
master: https://github.com/LimeSurvey/LimeSurvey/pull/3560

Please also see:
https://bugs.limesurvey.org/view.php?id=18880#c76944
https://github.com/LimeSurvey/LimeSurvey/pull/3433/commits/6e722025eacfeabb4f46b6cfdbda2957e0ae64a2

Basically this is the same as the fix on develop, backported to master and 5.x branches

Please note there will be merge conflicts between these PRs and the PRs for 19086

i have no way of knowing which will be merged first, so just ping me when they pop up and I will take care of it

to reproduce:
(debug>0)

after fix:

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=35666

Fix committed to 5.x branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=35667

Tested and merged.

Fixed in Release 5.6.43+231030