ListenableFuture#addListener with null executor throws NPE

[joedj]

ListenableFuture#addListener javadoc says:

* Executor can be <code>null</code>, in that case executor will be executed
* in the thread where completion happens.

However, this doesn't work with the NettyResponseFuture implementation:

java.lang.NullPointerException
at java.util.concurrent.CompletableFuture.screenExecutor(CompletableFuture.java:415)
at java.util.concurrent.CompletableFuture.whenCompleteAsync(CompletableFuture.java:2142)
at org.asynchttpclient.netty.NettyResponseFuture.addListener(NettyResponseFuture.java:257)

I'm happy to submit a pull request for this issue, if you can indicate how you want it fixed (i.e. update the documentation to reflect current behavour, or change behaviour to match documentation).

[slandelle]

Hi @joedj

Thanks for reporting! I guess it would be better to try to implement what the doc says. But if it turns out to be difficult, let's update the doc.
And sure, PR welcome :)