This is quite an edge case, and it might trigger an issue in the core.
Configure Kanban to list a category (or more, it doesn’t matter) on a single category (no child).
If you click on a topic and then navigate back, you get this error:
It happens only once. If you refresh the same board page, the same steps don’t trigger an error.
It doesn’t error with tags.
Running the latest Discourse version and Kanban.