Adding vertex streams in the inspector has stopped working.

Repro steps:
1. Open Scene1 from the attached project, or create a new particle system in a new project.
2. Select the particle system in the hierarchy.
3. In the inspector, enable vertex streams and try adding "Tangents"
4. Observe that the Tangent vertex stream cannot be added.

Notes:
- Regression, works in 2017.1.0f3
- Stopped working around 2017.2.0a4 and does not work in later versions
- Removing streams seems to work fine