Blender Git Loki

Fix Unreported: Run time error in UI code

Steps to reproduce:

1. Add clip to clip editor
2. Open the tracking settings & tracking settings extra panels

To fix this the sub panel is only drawn if a track is active.
The main panel will exit early and display a "No active track" message.
This is consistent with other panels in the clip editor.

Reviewed By: HooglyBoogly

Differential Revision: https://developer.blender.org/D9727