Topic management page: Cannot select topics for active or past event

[dan-du-car]

Summary

The topic management page allow users to filter the event based on timestamp, location, event status (Is Live event), testing type. However, the topic selection sections only can list available topics when the selected event at live event status, and there are units registered with this live event and provide available topics.
It is confusing to allow users to select past and active event, but no topics to choose.
In the below screenshot, the event is active, but not a live event and there is no unit currently registered to this event nor running:

Reasoning for new functionality

To make the UI more user friendly.