GUI throws ValueError

[tjduigna]

Describe the bug
Occasionally when using the Distance functionality in the GUI, depending on some previous interactions with the scene, the code will throw a ValueError which gets printed below the widget.

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/github/exatomic/exatomic/widgets/widget.py in _atom_df(c)
    504             c.value = not c.value
    505             if c.value:
--> 506                 self._update_output(out)
    507                 #link((self.scenes[0].selected, 'value'), (selected, 'value'))
    508             else:

~/github/exatomic/exatomic/widgets/widget.py in _update_output(self, out)
    471             idx[sdx] = [int(''.join(filter(lambda x: x.isdigit(), i))) for i in scn.selected['idx']]
    472             if len(idx[sdx])%2 != 0:
--> 473                 raise ValueError("Must select an even number of atoms. Last selected atom has been truncated.")
    474             atom_coords = self._df[sdx].atom.groupby('frame').get_group(scn.frame_idx). \
    475                                reset_index(drop=True).loc[[i for i in idx[sdx]], ['x', 'y', 'z']]

ValueError: Must select an even number of atoms. Last selected atom has been truncated.

To Reproduce
Relies on multiple GUI interactions (may be intermittent). Easier to reproduce when you have multiple universes in a single widget.

Expected behavior
I think that critical errors for the computational kernel that prevent functionality in the GUI should be handled and logged but not displayed to the front end.

Desktop (please complete the following information):

• OS: WS Linux 4.4.0-17134-Microsoft
• Browser: Chrome (Windows 10)
• Version: 73.0.3683.103

Additional context
This may only be reproducible in the case with multiple universes per UniverseWidget

[herbertludowieg]

I'll look into this. I think what I initially came up with was too unstable, @adamphil also showed some possible issues with the visualization in the UniverseWidget.

[herbertludowieg]

I think it might be a good idea for the time being to deprecate the distance stuff. I would like to fix it but don't have the time to do it right now.