Fix bug in building subpageNames list

[dplochcoder]

Summary of Changes

The subpageNames list should never be empty when passed to BuildSubtreeElements; a call to add the current name when recursing was missing.

Checklist

• No change is too small for a release, so pick one:
  • I have updated the package version following semantic versioning
  • There is another change actively WIP that will update the version (link issue or PR here)
  • This PR does not update user-facing code/config
  • I'm not sure how to set the version and would like the reviewer's help

[flibber-hk]

It worked with the BingoUI commit I presented