code: def get_next_step(self):
          
          layer_purpose = self.parent.step_kw_purpose.selected_purpose()
          if layer_purpose['key'] != layer_purpose_aggregation['key']:
              subcategory = self.parent.step_kw_subcategory. \
                  selected_subcategory()
          else:
              subcategory = {'key': None}
  
          # We don't use field mapping for populated place exposure. Use the
          # population point instead.
  
          # inasafe_fields = self.get_inasafe_fields()
          # If population field is set, must go to field mapping step first.
          # if population_count_field['key'] in inasafe_fields.keys():
          #     return self.parent.step_kw_fields_mapping
  
          # Check if it can go to inasafe default field step
          default_inasafe_fields = get_fields(
              layer_purpose['key'],
              subcategory['key'],
              replace_null=True,
              in_group=False)
          if default_inasafe_fields:
              return self.parent.step_kw_default_inasafe_fields
  
          # Any other case
          return self.parent.step_kw_source
docs: Find the proper step when user clicks the Next button.
  
          :returns: The step to be switched to
          :rtype: WizardStep instance or None