Improve robusness

Author: jfarcand

src/main/java/com/ning/http/client/providers/netty/NettyAsyncHttpProvider.java

@@ -2402,7 +2402,8 @@ public void onClose(ChannelHandlerContext ctx, ChannelStateEvent e) {
                 NettyWebSocket webSocket = NettyWebSocket.class.cast(h.onCompleted());
                 h.resetSuccess();
 
-                if (!(ctx.getAttachment() instanceof DiscardEvent))
+                log.trace("Connection was closed abnormally (that is, with no close frame being sent).");
+                if (!(ctx.getAttachment() instanceof DiscardEvent) && webSocket != null)
                     webSocket.close(1006, "Connection was closed abnormally (that is, with no close frame being sent).");
             } catch (Throwable t) {
                 log.error("onError", t);