The Dropdown icon for "Advanced" properties doesn't work under "Control Playable Asset" in Inspector.
Please refer to the attached video.

Steps to Reproduce:
1. Create a new project
2. GameObject > 3D Object > Cube
3. Window > Sequencing > Timeline
4. Create a Playable Director and Timeline Asset for Cube
5. Create Control Track and add Control Clip from GameObject
6. Inspector > Control Playable Asset > Advanced

Actual Result:
"Advanced" property dropdown icon is unresponsive OnClick.

Expected Result:
"Advanced" property dropdown icon should respond and open.

Workaround:
Clicking on the "Advanced" label instead of the dropdown icon opens up properties.

Occurring on:
2020.1.0a18, 2019.3.0f5, 2019.3.0a7

Working Fine on:
2019.3.0a6, 2019.2.17f1

Tested on Package: Timeline (Preview 5)1.3.0, 1.2.9

Environment:
Windows and Mac

License type: Free