Fix site launch flow after switching to a new site

[razvanpapadopol]

Changes proposed in this Pull Request

• Update RegisterDomainStep to prevent crashing when trying to launch a site after a site change.

Testing instructions

• Login to an account with at least 2 unlaunched sites. Sometimes the behaviour explained in Site launch flow broken after switching to a new site #42515 is causing the launch flow to crash.
• Launching a site after switching to a new site should always work.

Fixes #42515
Unblocks #42513

[matticbot]

Test live: https://calypso.live/?branch=fix/launch-flow-on-site-change

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~4 bytes added 📈 [gzipped])

name     parsed_size           gzip_size
domains        +14 B  (+0.0%)       +4 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Async-loaded Components (~4 bytes added 📈 [gzipped])

name                             parsed_size           gzip_size
async-load-signup-steps-domains        +14 B  (+0.0%)       +4 B  (+0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[sirreal]

This seems to fix the cited issue. I didn't find any regressions in my testing. Thanks!