Bug report: Application crashed with TraitError

## Automated report

_This issue was created with the app's automated bug reporting feature.
Attached to this issue is the full traceback as well as an environment
fingerprint that contains information about the operating system as well as all
installed libraries._

## Additional comments (optional):

_Example: I submitted a band structure calculation for Silica._

## Attachments

Traceback

```python-traceback
/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in set(self, obj, value)
704
705 def set(self, obj, value):
--> 706 new_value = self._validate(obj, value)
707 try:
708 old_value = obj._trait_values[self.name]

/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in _validate(self, obj, value)
738 value = self.validate(obj, value)
739 if obj._cross_validation_lock is False:
--> 740 value = self._cross_validate(obj, value)
741 return value
742

/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in _cross_validate(self, obj, value)
744 if self.name in obj._trait_validators:
745 proposal = Bunch({"trait": self, "value": value, "owner": obj})
--> 746 value = obj._trait_validators[self.name](obj, proposal)
747 elif hasattr(obj, "_%s_validate" % self.name):
748 meth_name = "_%s_validate" % self.name

/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in __call__(self, *args, **kwargs)
1231 """Pass `*args` and `**kwargs` to the handler's function if it exists."""
1232 if hasattr(self, "func"):
-> 1233 return self.func(*args, **kwargs)
1234 else:
1235 return self._init_call(*args, **kwargs)

/opt/conda/lib/python3.9/site-packages/ipywidgets/widgets/widget_selection.py in _validate_value(self, proposal)
241 return findvalue(self._options_values, value, self.equals) if value is not None else None
242 except ValueError:
--> 243 raise TraitError('Invalid selection: value not found')
244
245 @observe('value')

TraitError: Invalid selection: value not found
```

Environment fingerprint

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9LAr

**By submitting this issue I confirm that I am aware that this information can
potentially be used to determine what kind of calculation was performed at the
time of error.**