Fatal error in signup epilogue

[sentry-io]

Sentry Issue: JETPACK-IOS-GBN

EXC_BREAKPOINT: WordPressAuthenticator/LoginViewController.swift:132: Fatal error

  File "LoginViewController.swift", in LoginViewController.showSignupEpilogue
  ?, in GoogleSignupConfirmationViewController
  File "GoogleAuthenticator.swift", line 462, in GoogleAuthenticator.accountCreated
  File "GoogleAuthenticator.swift", line 428, in GoogleAuthenticator.createWordPressComUser
  File "SignupService.swift", line 34, in SignupService.createWPComUserWithGoogle
...
(18 additional frame(s) were not displayed)

[twstokes]

I was able to reproduce this by starting a login with Google and swiping back to the app landing screen before it completed loading. This presumably removes the navigationController that's expected here and causes the check to fail.