When having a repository with "projects" turned off, this header is null https://github.com/sindresorhus/refined-github/blob/173e39faf0fadc97e134d06d61911ebd86b418fb/source/features/clean-sidebar.tsx#L37-L43
but there is only a null check for nextElementSibling and not header

A link to a repo is not so valuable as you have to be an owner of the repo in order to see this bug. 😛