Subiquity can't establish connection after a new server controller has been added

I'm trying to add a new screen to Subiquity but Subiquity gets stuck every time I run it.

running server pid 40812

connecting... /

To create my controller I used an example from DESIGN.md My server controller class is as below:

import logging

from subiquity.common.apidef import API
from subiquity.server.controller import SubiquityController

log = logging.getLogger('subiquity.server.controllers.example')


class ExampleController(SubiquityController):

    endpoint = API.example
    model_name = 'example'

    async def GET(self) -> str:
        return self.model.thing

    async def POST(self, data: str):
        self.model.thing = data
        await self.configured()

Also, I added my Example controller class to controllers[] in subiquity/server/server.py:

controllers = [
        # other classes
        "Kernel",
        "Keyboard",
        "Example",
        "Zdev",
        "Source",
        # some other classes
        ]

And added example = simple_endpoint(str) to apidef.py

My model as simple as possible:

class ExampleModel:
    thing = "example_var"

What might cause the problem? If I remove my Example controller from controllers[], Subiquity works but obviously doesn't use my controller.