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