Bug report: Application crashed with AttributeError

## 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 _notify_trait(self, name, old_value, new_value)
1503
1504 def _notify_trait(self, name, old_value, new_value):
-> 1505 self.notify_change(
1506 Bunch(
1507 name=name,

~/.local/lib/python3.9/site-packages/ipywidgets/widgets/widget.py in notify_change(self, change)
699 # Send new state to front-end
700 self.send_state(key=name)
--> 701 super().notify_change(change)
702
703 def __repr__(self):

/opt/conda/lib/python3.9/site-packages/traitlets/traitlets.py in notify_change(self, change)
1515 def notify_change(self, change):
1516 """Notify observers of a change event"""
-> 1517 return self._notify_observers(change)
1518
1519 def _notify_observers(self, event):

/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_widgets_base/wizard.py in _observe_selected_index(self, change)
192 def _observe_selected_index(self, change):
193 "Activate/deactivate the next-button based on which step is selected."
--> 194 self._update_buttons()
195
196 def can_reset(self):

~/.local/lib/python3.9/site-packages/aiidalab_widgets_base/wizard.py in _update_buttons(self)
211 index = self.accordion.selected_index
212 if index is None:
--> 213 self.back_button.disabled = True
214 self.next_button.disabled = True
215 self.reset_button.disabled = True

AttributeError: 'WizardAppWidget' object has no attribute 'back_button'
```

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.**