def _validate_existence(self, value):
        if value in self.empty_values and self.required:
            raise ValidationError('this field is required')
Deletes multiple instances and makes sure the MPTT fields get
dataset/ETHPy150Open wuher/devil/devil/fields/fields.py/NestedField._validate_existence--crossover with--dataset/ETHPy150Open feincms/feincms/feincms/admin/tree_editor.py/TreeEditor.delete_selected_tree