Jetpack: Customizer: "we're sorry, an unexpected error has occurred" shown with eCommerce plans

[worldomonation]

Steps to reproduce the behavior

1. Create new site with Leven theme.
2. Upgrade to eCommerce plan.
3. Click on Appearance > Customizer.

What I expected to happen

Customizer loads without error.

What actually happened

Splash with We're sorry, an unexpected error has occurre is shown.
The page reloads automatically and the Customizer is then loaded.

Context

User scenario manual testing.

Browser / OS version

OS: macOS Catalina 10.15.7
Browser: Firefox Nightly 86.0a1, Chrome 88.0.4324.96 (Official Build) (x86_64)

Is this specific to the applied theme? Which one?

Tested with Leven.

Does this happen on simple or atomic sites or both?

Atomic only.
Tested with Leven on Simple site, was not reproducible.

Is there any console output or error text?

Many errors are shown on console, mostly relating to chunk load failure (specific chunk number changes each time):
```Uncaught (in promise) ChunkLoadError: Loading chunk 55400 failed.
(error: https://wordpress.com/calypso/evergreen/async-load-calypso-blocks-inline-help.2727b2f7b1f3c9637042.min.js)
j https://wordpress.com/customize/outrageouslyclassicrecords.wpcomstaging.com?retry=1:38
e https://wordpress.com/customize/outrageouslyclassicrecords.wpcomstaging.com?retry=1:38
e https://wordpress.com/customize/outrageouslyclassicrecords.wpcomstaging.com?retry=1:38
$t https://wordpress.com/calypso/evergreen/entry-main.dea226c06cb35f9703a7.min.js:2
require https://wordpress.com/calypso/evergreen/39821.222cc620e84221f28ae9.min.js:1
componentDidMount https://wordpress.com/calypso/evergreen/39821.222cc620e84221f28ae9.min.js:1
aa https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
vl https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
unstable_runWithPriority https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
qo https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
yl https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
rl https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
Xa https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
Ul https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
Wl https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
sl https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
Wl https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
render https://wordpress.com/calypso/evergreen/90402.d45dc3f6367787a5b625.min.js:2
AppBoot https://wordpress.com/calypso/evergreen/entry-main.dea226c06cb35f9703a7.min.js:2
AppBoot https://wordpress.com/calypso/evergreen/entry-main.dea226c06cb35f9703a7.min.js:2
promise callback*./boot/app.js/window.AppBoot/ AppBoot https://wordpress.com/calypso/evergreen/entry-main.dea226c06cb35f9703a7.min.js:2
AppBoot https://wordpress.com/calypso/evergreen/entry-main.dea226c06cb35f9703a7.min.js:2
https://wordpress.com/customize/outrageouslyclassicrecords.wpcomstaging.com?retry=1:38

lastly:

GEThttps://outrageouslyclassicrecords.wpcomstaging.com/wp-admin/undefined
[HTTP/2 404 Not Found 266ms]
```

Level of impact (Does it block purchases? Does it affect more than just one site?)

Moderate - not a good experience for eCommerce plan users.

Screenshot / Video: If applicable, add screenshots to help explain your problem.

https://user-images.githubusercontent.com/6549265/106077900-46f2ae80-60c7-11eb-86fa-eb0df912f7f2.mov

[cathymcbride]

@obenland would this be related to nav unification?

[obenland]

Possibly. https://outrageouslyclassicrecords.wpcomstaging.com/wp-admin/undefined has me thinking yes, but I'm not aware of nav unification changing the way the Customizer loads in Calypso.

[obenland]

I can reproduce it on my testing Atomic site as well, clicking on Appearance > Customize in wp-admin.

[obenland]

It looks like we link directly to wp-admin's Customizer for WoA sites, which loads without issue for me. We should change that in Nav Unification.

[obenland]

@getdave @cpapazoglou Can we try to get this into Jetpack 9.4 still?