Motivation:
SslHandler adds a pending write with an empty buffer and a VoidChannelPromise when a user flush and not pending writes are currently stored. This may produce an IllegalStateException later if the user try to add a ChannelFutureListener to the promise in the next ChannelOutboundHandler.
Modifications:
Replace ctx.voidPromise() with ctx.newPromise()
Result:
No more IllegalStateException possible