Some smaller things

[KillTheMule]

1. This should check for nil also, especially for the sync_request usecase where people probably don't register a callback often.
2. Sync requests are async requests with a (busy?) waiting loop. You're waiting 100ms in each step... isn't that much too much?
3. Please consider doing this. Note that this is somewhat bad if value is nil, I suggested something about verification in another issue.