The x means the topic doesn't exist in the project. If you deleted the source field I'd have thought that should unlink the section (I can't try it myself right now), so it could be a bug. I'd close and reopen the TOC and see if the source field is still blank. Then try to reproduce in a test project.
The quickest way to fix it is probably to create a new section without linking to a topic, then drag the topics under the new section.