Fix compilation warnings for MapBuffer

Summary:
We have two warnings on MapBuffer which I'd like to resolve as they show up on console every time.
1. We're using a deprecated method receiveCommand, which is actually legit but was missing a propagation of the warning. I'm adding it.
2. We had a null check that that was always not null. That is enforced by the Kotlin type system. I've checked the code and
we're actually always returning non-nulls there or raising exceptions instead.

Changelog:
[Internal] [Changed] - Fix compilation warnings for MapBuffer

Reviewed By: javache

Differential Revision: D41522129

fbshipit-source-id: c2dbb660f95a2ff7dac6e4fcdf476e4058cf730e