[layouts] Fix crash when opening layout with dynamic image in html label

[nyalldawson]

Fixes #28996

[backporting]

The backport to release-3_4 failed:

Commits ["f057354ec899b28a7ee27d4b8990b992d7d0b6d5"] could not be cherry-picked on top of release-3_4

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport release-3_4
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick f057354ec899b28a7ee27d4b8990b992d7d0b6d5
# Create a new branch with these backported commits.
git checkout -b backport-31880-to-release-3_4
# Push it to GitHub.
git push --set-upstream origin backport-31880-to-release-3_4
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is release-3_4 and the compare/head branch is backport-31880-to-release-3_4.