[Crash] Crash when putting the app into background while retrieving server configuration

### Steps to reproduce

1. Start the create account flow
2. Open the screen to edit the homeserver
3. Click on NEXT you can keep `matrix.org`
4. While loading, put the app into background

### Outcome

#### What did you expect?

No crash, and putting the app back to foreground should restore the state.

#### What happened instead?

Crash running this code:
```kotlin
OnboardingViewEvents.OnHomeserverEdited -> {
supportFragmentManager.popBackStack( // Crash here
FRAGMENT_EDIT_HOMESERVER_TAG,
FragmentManager.POP_BACK_STACK_INCLUSIVE
)
ensureEditServerBackstack()
}
```

Stacktrace:

```
java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState
at androidx.fragment.app.FragmentManager.checkStateLoss(FragmentManager.java:1610)
at androidx.fragment.app.FragmentManager.enqueueAction(FragmentManager.java:1650)
at androidx.fragment.app.FragmentManager.popBackStack(FragmentManager.java:795)
at im.vector.app.features.onboarding.ftueauth.FtueAuthVariant.handleOnboardingViewEvents(FtueAuthVariant.kt:235)
```

### Your phone model

Any

### Operating system version

Any

### Application version and app store

1.5.2 and probably older

### Homeserver

NA

### Will you send logs?

No

### Are you willing to provide a PR?

No