Usage of CellSize filter

Dear community,
I simulate bubble collapses.
With a pvbatch-script I want to calculate the volume of the bubble.
I use the precompiled version ParaView-5.10.0-RC1-osmesa-MPI-Linux-Python3.9-x86_64
The following function should do this:

def Process( file ):
   xdmf = pv.Xdmf3ReaderS( FileName = [file] )
   xdmf.CellArrays = ['levelset' ]
   bubble = createThreshold( xdmf )
   v = computeCellSize( bubble )
   i = np.sum(v)
   return i

The ´createThreshold´ function creates a dataset which contains only the bubble, therefore the name of its result. The computeCellSize function looks like this:

def computeCellSize( input_data ):
   cs = pv.CellSize(registrationName='CellSize1', Input=input_data)
   cs.ComputeVertexCount = 0
   cs.ComputeLength = 0
   cs.ComputeArea = 0
   size = cs.CellData.GetArray('Volume')
   return size

It returns the volume array to the Process function. However, the i = np.sum(v) line only returns None values.
What is the reason for this?

UpdatePipeline ?

I added UpdatePipeline before the sum-line, but I keep getting None-values

size = cs.CellData.GetArray('Volume')

This is wrong, you need to fetch the data from the server first.

Could you tell me how I can do that?

Something like:

   cs.ComputeArea = 0
   dataset = servermanager.Fetch(cs)
   size = dataset.GetCellData().GetArray('Volume')