I'm unable to clear the bone mapping from the rig mapping menu.

Steps to reproduce
- Open attached project
- Select Assets > Animations > ReleaseQA > Models > Release QA - Winston
- Unlock the inspector and select the rig tab.
- Click Configure
- Under mapping find the mapping dropdown and select clear
- The bones are now cleared.
- Select apply
Notice Issue > The bones are not cleared.

Expected: All bone slots should be empty.

Broken: 2018.3.0a7, 2018.3.0a9, 2018.3.0b1, 2018.3.0b5, 2018.3.0b12, 2018.3.0f2, 2018.3.12f1, 2019.1.0b10,2019.2.0a9
Working: 2018.1.9f2, 2018.2.10f1, 2018.3.0a1, 2018.3.0a3, 2018.3.0a4, 2018.3.0a5, 2018.3.0a6