How to reproduce:
1. Lock the inspector panel
2. Attempt to create a Polyshape- can't do it.
3. Unlock the inspector panel - now I can create Polyshapes again.

Expected result: You can create new points for Poly Shape.
Actual result: You cannot create new points for Poly Shape.

Reproducible with - 2019.1.0a8, 2018.3.0b10, 2018.2.16f1, 2018.1.9f2, 2018.1.0a1
Not reproducible with - 2017.4.15f1, 2017.4.0f1

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This is not a ProBuilder issue, but rather a problem with the Inspector tracker. There is a case here https://fogbugz.unity3d.com/f/cases/1104794/, and a PR in the merge queue here https://ono.unity3d.com/unity/unity/pull-request/79036/_/ui-elements/inspector-element/store-editor-reference .