bug(at_onboarding_flutter): passing an explicit atSign which isn't in the keychain shouldn't throw an error

[XavierChanth]

See video explanation:

CleanShot.2024-10-16.at.14.42.40.mp4

Summary:

If I explicitly pass an atSign which is not currently in the keychain to AtOnboarding.onboard. It should check the status of that atSign and defer to the appropriate screen. Instead, it fails with an error.

[XavierChanth]

@sitaram-kalluri here's the scenario:

AtOnboarding.onboard will not accept an explicit atSign if the atSign is not in the keychain. It expects null for the atSign argument if the atSign is not in the keychain.

This is a problem in NoPorts Desktop since we wrap onboarding with a custom picker/form that allows users to manage atSigns in multiple root domains.

What I expect AtOnboarding.onboard to do:

1. If the atSign is null, use the existing behavior
2. If the atSign is already in the keychain, just do onboarding like normal (no work to do here)
3. If that atSign is not in the keychain we should detect whether the atSign is activated or not:
4. If the atSign is not activated, I expect that the widget skips past the normal set of screens
   including the screen where you input the atSign to activate, and goes straight into the activate
   flow
5. If the atSign is activated, I expect it to show options to either:
   a) upload the key file
   b) do the apkam onboarding flow

Based on the way that the widget is currently designed it may be complicated to implement 3,4,5 separately
I think that it is also fine, but less preferred, if this is one screen with 3 buttons. But, the activate button
in this scenario should still bypass the screen that asks for the atSign to activate, since that has already been provided.