Label autocomplete dropdown temporarily displays "undefined" as the only choice when first opened

regression

Summary

When using the autocomplete dropdown to autocomplete a snippet reference (e.g. ~regression), the dropdown will display undefined until the ajax request for the list of labels finishes.

Steps to reproduce

1. Go to the issue creation page.
2. In the issue description box, type ~.

Expected behavior

The dropdown should appear with a loading icon and label (similar to issue and MR references) before the server responds with the relevant labels.

Actual behavior

undefined is displayed before the label listing request completes.

Relevant logs and/or screenshots

Output of checks

GitLab.com 8.16.0-rc3-ee 4ec848c