Fix unexpected error thrown if instance_domain is not set

[robbiet480]

Fixes this error that mobile_app users that don't use cloud are getting, which causes a 500 and registration to fail:

Log Details (ERROR)
Wed May 01 2019 21:32:02 GMT-0400 (Eastern Daylight Time)
Error handling request
Traceback (most recent call last):
  File "/srv/homeassistant/lib/python3.6/site-packages/aiohttp/web_protocol.py", line 418, in start
    resp = await task
  File "/srv/homeassistant/lib/python3.6/site-packages/aiohttp/web_app.py", line 458, in _handle
    resp = await handler(request)
  File "/srv/homeassistant/lib/python3.6/site-packages/aiohttp/web_middlewares.py", line 119, in impl
    return await handler(request)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/http/real_ip.py", line 33, in real_ip_middleware
    return await handler(request)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/http/ban.py", line 68, in ban_middleware
    return await handler(request)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/http/auth.py", line 216, in auth_middleware
    return await handler(request)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/http/view.py", line 115, in handle
    result = await result
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/http/data_validator.py", line 46, in wrapper
    result = await method(view, request, *args, **kwargs)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/mobile_app/http_api.py", line 57, in post
    remote_ui_url = async_remote_ui_url(hass)
  File "/srv/homeassistant/lib/python3.6/site-packages/homeassistant/components/cloud/__init__.py", line 125, in async_remote_ui_url
    return "https://" + hass.data[DOMAIN].remote.instance_domain
TypeError: must be str, not NoneType

Thanks to @arsaboo

home-assistant/iOS#299

[ghost]

Hey there @home-assistant/core, mind taking a look at this pull request as its been labeled with a integration (cloud) you are listed as a codeowner for? Thanks!

This is a automatic comment generated by codeowners-mention to help ensure issues and pull requests are seen by the right people.

[andrewsayre]

Should this be tagged for 0.92.2?