Hello,

I want to read in a VTK file, evaluate some function on it and write the file back, including the function result. All that, using the Python VTK interface.

So far I got:

```
from vtk import *
reader = vtkDataSetReader()
reader.SetFileName("bunny-0.1.vtk")
reader.ReadAllVectorsOn()
reader.ReadAllScalarsOn()
reader.Update()
data = reader.GetOutput()
calc = vtkArrayCalculator()
calc.SetInputData(data)
calc.SetFunction("5")
calc.SetResultArrayName("MyResults")
calc.Update()
# Gives: AttributeError: 'NoneType' object has no attribute 'GetPointData'
# print(calc.GetPolyDataOutput().GetPointData().GetArray("MyResults").getValue(10))
writer = vtkUnstructuredGridWriter()
writer.SetInputData(data)
writer.SetFileName("Output.vtk")
writer.Write()
```

It works so far, that it writes out the same geometry to Output.vtk, but without the additional field (here, constant 5).

What is the missing piece?

Thanks!