A typo and some feedback on API design

` guard error != nil else {
// handle login error
return
}`
I think you mean `guard error == nil else {...}`.

Regarding API design, it's tedious.
You have to check error every step of the way. Singleton may not even exist at all.
No point for nested login, e.g; loadLoginSession followed by login, you are exposing your implementation details for an opportunity at error handling that no one cares.

Nil coalescing to make singleton non-optional, and make calls chainable, e.g.;
`GrabIdPartner.shared.login(presentingViewController: vc).onSuccess { session in ...}.onFailure { error in ...} `

User does not care where you go wrong, and I only care for debug purpose. Provide some debug logging instead of repeated nested unwrapping of both your optionals and weak selfs.

By the way, I'll just inject dependency injection, e.g.; `self?.callback(...)` instead of `guard let self = self`,
it looks more professional.

Start with the login guard shown in the issue and the loadLoginSession/login entry points. Review the proposed chaining, optional handling, dependency injection, and debug logging against the existing SDK API. Done requires an agreed design and implementation scope, but the issue names no files or tests.

Written by the indexing model from the issue text.