FiPy: 'AssignAttribute' object has no attribute 'cell_data'

I'm trying to run the following example, taken from: https://www.ctcms.nist.gov/fipy/fipy/generated/fipy.viewers.mayaviViewer.html
``` python
import os
os.environ["QT_API"] = "pyqt"

from traits.etsconfig.api import ETSConfig
ETSConfig.toolkit = 'qt4'

from fipy import *
mesh = Grid3D(nx=50, ny=100, nz=10, dx=0.1, dy=0.01, dz=0.1)
x, y, z = mesh.cellCenters
xyzVar = CellVariable(mesh=mesh, name=r"x y z", value=x * y * z)
k = Variable(name="k", value=0.)
viewer = MayaviClient(vars=numerix.sin(k * xyzVar),
limits={'ymin': 0.1, 'ymax': 0.9},
datamin=-0.9, datamax=2.0,
title="MayaviClient test")
from builtins import range
for kval in range(10):
k.setValue(kval)
viewer.plot()
viewer._promptForOpinion()
```
However, I'm getting the following traceback:
```
Jul 08 13:54:38 linux compiz[2909]: Traceback (most recent call last):
Jul 08 13:54:38 linux compiz[2909]: File "/usr/lib/python2.7/dist-packages/pyface/ui/qt4/gui.py", line 183, in event
Jul 08 13:54:38 linux compiz[2909]: self._callable(*self._args, **self._kw)
Jul 08 13:54:38 linux compiz[2909]: File "~/.local/lib/python2.7/site-packages/fipy/viewers/mayaviViewer/mayaviDaemon.py", line 130, in run
Jul 08 13:54:38 linux compiz[2909]: if out.cell_data.scalars is not None]
Jul 08 13:54:38 linux compiz[2909]: AttributeError: 'AssignAttribute' object has no attribute 'cell_data'
```
How shall I construct the code to be Mayavi properly called?

Start with the MayaviClient example in the issue and inspect mayaviDaemon.py around line 130, where the traceback accesses out.cell_data.scalars. Reproduce the example with the reported Python 2.7 and Qt setup, then verify that the viewer can plot the FiPy data without the AttributeError.

Written by the indexing model from the issue text.