Bug report: Application crashed with UnboundLocalError

## 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):

Was testing hubbard parameters. Accidentally selected Si. Went back and selected Au, which triggered the error.

## Attachments

Traceback

```python-traceback
/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in _notify_observers(self, event)
1562 c = getattr(self, c.name)
1563
-> 1564 c(event)
1565
1566 def _add_notifiers(self, handler, name, type):

~/.local/lib/python3.9/site-packages/aiidalab_qe_muon/app/configuration/view.py in _on_input_structure_change(self, _)
403 def _on_input_structure_change(self, _):
404 self.refresh(specific="structure")
--> 405 self._model.on_input_structure_change()
406 self._model.compute_suggested_supercell()
407 if hasattr(self, "why_no_pol_text"):

~/.local/lib/python3.9/site-packages/aiidalab_qe_muon/app/configuration/model.py in on_input_structure_change(self, _)
237 self.reset()
238 else:
--> 239 self.check_polarization_allowed()
240 self.disable_x, self.disable_y, self.disable_z = True, True, True
241 pbc = self.input_structure.pbc

~/.local/lib/python3.9/site-packages/aiidalab_qe_muon/app/configuration/model.py in check_polarization_allowed(self, _)
222 # if no isotopes are found, the calculation will fail, so don't allow to run it.
223 if self.input_structure:
--> 224 info, isotopes, isotope_list = check_enough_isotopes(self.input_structure.get_ase())
225 if len(isotope_list) == 0:
226 self.polarization_allowed = False

~/.local/lib/python3.9/site-packages/undi/undi_analysis.py in check_enough_isotopes(structure, abundance_threshold, spin_threshold, without_muon)
40 candidate_isotopes.append(isotope)
41
---> 42 return info, isotopes, candidate_isotopes
43
44

UnboundLocalError: local variable 'isotopes' referenced before assignment
```

Environment fingerprint

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time of error.**