Minor enhancement to avoid null exceptions.

[iPazooki]

In the AwaitableFactory.cs file, the TryGet method may return null, which is not considered good practice. To address this issue and prevent potential null reference exceptions, I recommend changing the return type to IAwaitableFactory?. This indicates that the return value can be nullable, alerting developers that they need to handle this scenario appropriately. Subsequently, we need to change five more places because of this change which is not a big deal.