The Dropdown icon for "Contacts" properties doesn't work under the "Polygon Collider 2D" component in Inspector.

Steps to repro:
1. Create a new project
2. Hierarchy >right-click > Create Empty
3. Inspector > Add Component > Polygon Collider 2D
4. Click > Contacts expand icon

Actual Result:
"Contacts" Dropdown icon is not clickable.

Expected Result:
"Contacts" Dropdown icon should be click and expand.

Reproducible in:
2020.1.0a18, 2019.3.0f5 (209f7fb30bd5), 2019.3.0a7

Working fine in:
2019.3.0a6, 2019.2.14f1

Environment:
Window 10, macOS 10.14