Bug Report: Open Materials Database inaccessible

## 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):

Trying to select "Open Materials Database" from the OPTIMADE client results in this error.

## Attachments

Traceback

```python-traceback
/opt/conda/lib/python3.7/site-packages/traitlets/traitlets.py in set(self, obj, value)
578
579 def set(self, obj, value):
--> 580 new_value = self._validate(obj, value)
581 try:
582 old_value = obj._trait_values[self.name]

/opt/conda/lib/python3.7/site-packages/traitlets/traitlets.py in _validate(self, obj, value)
612 value = self.validate(obj, value)
613 if obj._cross_validation_lock is False:
--> 614 value = self._cross_validate(obj, value)
615 return value
616

/opt/conda/lib/python3.7/site-packages/traitlets/traitlets.py in _cross_validate(self, obj, value)
618 if self.name in obj._trait_validators:
619 proposal = Bunch({'trait': self, 'value': value, 'owner': obj})
--> 620 value = obj._trait_validators[self.name](obj, proposal)
621 elif hasattr(obj, '_%s_validate' % self.name):
622 meth_name = '_%s_validate' % self.name

/opt/conda/lib/python3.7/site-packages/traitlets/traitlets.py in __call__(self, *args, **kwargs)
975 """Pass `*args` and `**kwargs` to the handler's function if it exists."""
976 if hasattr(self, 'func'):
--> 977 return self.func(*args, **kwargs)
978 else:
979 return self._init_call(*args, **kwargs)

/opt/conda/lib/python3.7/site-packages/ipywidgets/widgets/widget_selection.py in _validate_index(self, proposal)
223 return proposal.value
224 else:
--> 225 raise TraitError('Invalid selection: index out of bounds')
226
227 @observe('index')

TraitError: Invalid selection: index out of bounds
```

Environment fingerprint

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**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.**