Replace receiveOrNull with receiveOrClosed once released.

[zach-klippenstein]

Starting in 1.3.0-RC, receiveOrClosed exists – however it's marked as @InternalCoroutinesApi. There are plans to make it public, but it's blocked by inline class stability (Kotlin/kotlinx.coroutines#1343 (comment)).

[zach-klippenstein]

Tried using the preview version of receiveOrClosed, but turns out it's broken (Kotlin/kotlinx.coroutines#1584) and caused a set of bugs with odd stacktraces (#626).

[zach-klippenstein]

The referenced coroutines bug was fixed some time ago. Might want to give this another try.