Blender Git Loki

Fix T72647: Check if the PBVH type makes sense for the sampling mode

Before this it was possible to use the operator with Dyntopo sample mode
with a PBVH type GRIDS or FACES, causing a crash. Now we check first if
the PBVH type is correct before calling the sampling function.

We also check if the PBVH exists, which may also cause a crash.

Reviewed By: jbakker

Maniphest Tasks: T72647

Differential Revision: https://developer.blender.org/D6475