Update: the issue turned out to be the vtkVolume and vtkVolumeMapper. Those should be vtkActor and vtkMapper, since I’m not trying to render a volume but a smooth surface.
In my case, using the code in my original post, that still threw an error. This error was related to the lookuptable and piecewisefunction I used to color the volume (that I no longer had after applying smoothing). Removing the code to apply colors to a volume rendered a smooth surface.