Fix crash on value relation multi-column

[elpaso]

Fix #54164

[nyalldawson]

Is the problem that "item" returns null here? It might be cleaner to just go:

if ( currentItem = item( rowIndex, columnIndex) ) { ... }

[elpaso]

Yes, that's the problem, but I find it cleaner this way: why would we want to iterate over a non-existing table item when we already know in advance that the index is overflowing?

[nyalldawson]

Well the logic for sizing mCache and building checkbox widgets based on the size of mCache is somewhere else entirely, and there's a good chance that if someone messes with that logic they won't realise that this code even exists and needs updating accordingly.

[qgis-bot]

The backport to release-3_28 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 2341d3f5810... Fix crash on value relation multi-column
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

stdout

Auto-merging src/gui/editorwidgets/qgsvaluerelationwidgetwrapper.cpp
CONFLICT (content): Merge conflict in src/gui/editorwidgets/qgsvaluerelationwidgetwrapper.cpp

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_28 release-3_28
# Navigate to the new working tree
cd .worktrees/backport-release-3_28
# Create a new branch
git switch --create backport-54889-to-release-3_28
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 2341d3f58104012263d64fcf809007e0a736fee3
# Push it to GitHub
git push --set-upstream origin backport-54889-to-release-3_28
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_28

Then, create a pull request where the base branch is release-3_28 and the compare/head branch is backport-54889-to-release-3_28.