Cell.plot does not have any math sanity checks

[MicahGale]

Bug Description

When plotting a Null region (defined as the Null set) cell.plot tries to do non-sensical math, and raises a nonhelpful error. There sound be some sanity checking before hand that raises a helpful error stating that there is nothing to plot, etc.

Steps to Reproduce

Note these actions only currently work due to another bug #3016.

import openmc
region = -openmc.model.CylinderSector(1.0, 1.1, 90, 95, (10, 0))
cell = openmc.Cell(region=region)
cell.plot

This leads to the error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[5], line 1
----> 1 cell.plot()

File ~/mambaforge/lib/python3.10/site-packages/openmc/cell.py:624, in Cell.plot(self, *args, **kwargs)
    622 u = openmc.Universe(cells=[self], universe_id=openmc.Universe.next_id + 1)
    623 openmc.Universe.used_ids.remove(u.id)
--> 624 return u.plot(*args, **kwargs)

File ~/mambaforge/lib/python3.10/site-packages/openmc/universe.py:411, in Universe.plot(self, origin, width, pixels, basis, color_by, colors, seed, openmc_exec, axes, legend, axis_units, legend_kwargs, outline, **kwargs)
    409 if isinstance(pixels, int):
    410     aspect_ratio = width[0] / width[1]
--> 411     pixels_y = math.sqrt(pixels / aspect_ratio)
    412     pixels = (int(pixels / pixels_y), int(pixels_y))
    414 axis_scaling_factor = {'km': 0.00001, 'm': 0.01, 'cm': 1, 'mm': 10}

ValueError: math domain error

Environment

• openmc: 0.14.0
• python: 3.10.13

[paulromano]

I'm unable to reproduce on the develop branch currently. @MicahGale if this continues to be a problem for you, please re-raise or file a new issue.