Auto-completed topics are off-by-1 when added to the list

Steps to reproduce:

1. Make sure you have a list of 3 or 4 topics
2. Edit a song, and go to the authors/topics tab
3. Click the down arrow to show the list of topics
4. Select a topic
5. Click Add button
6. See that correct topic appears in the list below
7. Click in the edit box of the combo box
8. Start typing a topic, and select one of the suggested topics (mouse or keyboard)
9. Click the Add button
10. See that the incorrect topic is added (the one below the selected topic)

Note: This may only occur on macOS.

See https://support.openlp.org/scp/tickets.php?id=3889 (video)