Bug report: Application crashed with TraitError (OPTIMADE MaterialsCloud provider)

## 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):

I selected the MaterialsCloud provider from the OPTIMADE structure importer.

## Attachments

Traceback

```python-traceback
/opt/conda/lib/python3.9/site-packages/ipyoptimade/subwidgets/provider_database.py in _observe_providers(self, change)
153 value = change["new"]
154 self.show_child_dbs.display = "none"
--> 155 self.provider = value
156 if value is None or not value:
157 self.show_child_dbs.display = "none"

/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in __set__(self, obj, value)
730 raise TraitError('The "%s" trait is read-only.' % self.name)
731 else:
--> 732 self.set(obj, value)
733
734 def _validate(self, obj, value):

/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)
736 return value
737 if hasattr(self, "validate"):
--> 738 value = self.validate(obj, value)
739 if obj._cross_validation_lock is False:
740 value = self._cross_validate(obj, value)

/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in validate(self, obj, value)
2149 return value
2150 else:
-> 2151 self.error(obj, value)
2152
2153 def info(self):

/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in error(self, obj, value, error, info)
842 describe("the", value),
843 )
--> 844 raise TraitError(e)
845
846 def get_metadata(self, key, default=None):

TraitError: The 'provider' trait of a ProviderImplementationChooser instance expected a LinksResourceAttributes or None, not the dict {}.
```

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.**