Note that to hit-test against the mesh you have to provide a your own FDynamicMeshAABBTree3. You do this by providing a callback via SetSpatialSource(). The reason for this is that (1) owners of an FMeshTopologySelector likely already have a BVTree and (2) if the use case is deformation, we need to make sure the owner has recomputed the BVTree before we call functions on it. The callback you provide should do that!