Steps to reproduce:
1. Open attached project
2. Open SimpleAnim scene
3. Select "Timeline" object and open Timeline window
4. Go into/out of Play mode a couple of times
5. You should see the timeline clips become transparent, timeline itself mostly unresponsive

Expected: Timeline should not lose style and become unresponsive
Actual: When the timeline breaks, can't scrub the timeline, can't select clips

No repro:
2019.1.14f1 Package Version 1.0.0, 2019.2.5f1 PV 1.1.0. 2019.3.0a4 PV 1.1.0, 2019.3.0a9 PV 1.1.0, 2019.3.0a10 PV 1.1.0, 2019.3.0a11 PV 1.1.0

Repro:
2019.3.0b3 Package Version 1.2.1, 2019.3.0b1 PV 1.1.0 (Timeline asset got corrupted when going into play mode), 2019.3.0a12 PV 1.1.0