Riot can get stuck endlessly pinging /register many times a second

[anoadragon453]

Description

After trying to create an account and failing because the ID already existed, Riot got into a state where it slammed /register many times a second.

Steps to reproduce

• Go to login screen
• Switch to Other as host
• Create account
• See that it's taken
• Switch to Matrix.org as host
• Switch back to Other as host
• Switch back to Sign-in

Logs being sent: yes

Version information

• Platform: web

For the web app:

• Browser: Firefox v69.0a1
• OS: Debian Stretch
• Version: Downloaded from buildkite: bba167c4332e-react-111b28971a24-js-a8d200dd02d5

[turt2live]

Judging from the logs, my theory is that we're not cleaning up the UIA instance correctly when the server change happens. It seems to be re-submitting the dummy request over and over, and not getting an error to break the loop. If it was properly cleaned up, it should break the loop immediately.

[t3chguy]

Closing in favour of #23084 which has more repro

Duplicate of #23084