[Probuilder/Polybrush] MissingReferenceException thrown when you have a polybrush brush and you modify Probuilder shape

To reproduce:
1. Create a new project with ProBuilder and Polybrush in it
2. Select a Polybrush brush
3. Click New Shape in ProBuilder
4. Mouse over the shape/click on it (so that Polybrush Mesh appears)
5. In New Shape dialog modify the shape
Expected result: The shape is modified cleanly.
Actual result: Shape is modified and MissingReferenceException is thrown

Reproduced on: 2019.3.0a10, with Polybrush 1.0.0 and ProBuilder 4.1.0