I have written an addon that creates a new dropdown question type (Vehicle Registration) in Survey. When the user runs the survey, they are presented with a dropdown to select a vehicle from a dynamically created list of the currently registered vehicles in the Fleet module. This bit works. However, when the user submits the survey, it crashes with the error:

File "/opt/odoo17/odoo17/addons/survey/controllers/main.py", line 536, in survey_submit
    answer_sudo._save_lines(question, answer, comment, overwrite_existing=survey_sudo.users_can_go_back or question.save_as_nickname or question.save_as_email)
  File "/opt/odoo17/odoo17/addons/survey/models/survey_user_input.py", line 301, in _save_lines
    raise AttributeError(question.question_type + ": This type of question has no saving function")
AttributeError: vehicle_dropdown: This type of question has no saving function

This is my survey_question.py:

from odoo import models, fields, api
import logging

_logger = logging.getLogger(__name__)

class SurveyQuestion(models.Model):
    _inherit = 'survey.question'
    question_type = fields.Selection(selection_add=[('vehicle_dropdown', 'Vehicle Dropdown')])
    fleet_vehicles = fields.Many2many('fleet.vehicle', string='Fleet Vehicles')

class SurveyUserInputLine(models.Model):
    _inherit = 'survey.user_input.line'

    @api.model
    def get_options(self):
        self.ensure_one()
        if self.question_id.question_type == 'vehicle_dropdown':
            return [(vehicle.id, vehicle.license_plate) for vehicle in self.question_id.fleet_vehicles]
        return super().get_options()

    def _save_lines(self, question, answer, comment=None, overwrite_existing=False):
        _logger.info(f"Saving lines for question: {question.id} ({question.question_type})")
        _logger.info(f"Answer: {answer}, Type: {type(answer)}")

        if question.question_type == 'vehicle_dropdown':
            if answer:
                try:
                    answer = int(answer)
                    _logger.info(f"Answer after int conversion: {answer}, Type: {type(answer)}")
                    existing_line = self.search([
                        ('user_input_id', '=', self.user_input_id.id),
                        ('question_id', '=', question.id)
                    ], limit=1)

                    if existing_line and overwrite_existing:
                        existing_line.write({'value_reference': answer})
                        _logger.info(f"Updated existing line {existing_line.id} with value_reference {answer}")
                    else:
                        self.create({
                            'user_input_id': self.user_input_id.id,
                            'question_id': question.id,
                            'answer_type': 'selection',
                            'value_reference': answer,
                        })
                        _logger.info(f"Created new line with value_reference {answer}")
                    return True
                except ValueError as e:
                    _logger.error(f"Could not convert answer to integer: {e}")
                except Exception as e:
                    _logger.exception(f"Error in _save_lines: {e}")
            else:
                _logger.info("No answer provided for vehicle dropdown")
        return super()._save_lines(question, answer, comment, overwrite_existing)

And this is my survey_question_views.xml:

<?xml version="1.0" encoding="UTF-8"?>
<odoo>
    <!-- Inherit the survey.question form view -->
    <record id="view_survey_question_form_inherit" model="ir.ui.view">
        <field name="name">survey.question.form.inherit</field>
        <field name="model">survey.question</field>
        <field name="inherit_id" ref="survey.survey_question_form"/>
        <field name="arch" type="xml">
            <!-- Add the Fleet Vehicles field after the question type field -->
            <xpath expr="//field[@name='question_type']" position="after">
                <field name="fleet_vehicles" widget="many2many_tags" options="{'no_create': True}" placeholder="Select vehicles"/>
            </xpath>
        </field>
    </record>
    <!-- Inherit the survey.question_container template -->
<template id="survey_vehicle_dropdown" inherit_id="survey.question_container">
        <!-- Target the inner div after the <h3> question title -->
        <xpath expr="//div[h3]" position="after">
            <t t-if="question.question_type == 'vehicle_dropdown'">
                <div class="form-group">
                    <label t-esc="question.title" class="form-label"/>
                    <select name="answer" class="form-control" required="1">
                        <option value="">Select a Vehicle</option>
                        <!-- Dynamically load only vehicles with 'Registered' status -->
                        <t t-foreach="env['fleet.vehicle'].search([('state_id', '=', 3)])" t-as="vehicle">
                            <option t-att-value="vehicle.id" t-esc="vehicle.license_plate"/>
                        </t>
                    </select>
                </div>
            </t>
        </xpath>
    </template>
</odoo>

Can anyone help where I'm going wrong?