How to reproduce:
1. Create a new project
2. Open Package Manager and go to Built-in packages
3. Disable Physics built-in package
4. Observe the Console

Expected result: Unity UI package can operate without the Physics built-in package
Actual result: Uninstalling built-in Physics module results in errors thrown by the Unity UI package

Reproducible with: 2019.2.0b10 (Unity UI 1.0.0), 2019.3.0a3 (Unity UI 1.0.0)

Notes:
- Could not test with versions, earlier than 2019.2.0b1 and 2019.3.0a3 as Unity UI package was not present.
- Before Unity UI became a package instead of a built-in package, the problem was not reproducible.
- Disabling Physics built-in package require other built-in packages being disabled (XR, VR, Vehicles, Terrain Physics, Cloth), however disabling these packages does not have any effect on Unity UI package.