I’m using PHP 8.0 and encountered this issue, but it might’ve happened in earlier versions. Either way, it looks like there’s a basic TypeError issue with a quick fix.

I’ve added a pull request with the proposed fix at: https://github.com/eduardozulian/admin-bar-color/pull/10

I’d love to see this fix implemented as these newer PHP versions are right around the corner (if not already there for some) with the fix then not being a hinderance (just adds a simple fix that does a check if a variable is empty or not to then avoid a TypeError).