RejectedExecutionException occurs during shutdown if an open websocket session exists

Issue

The following message is logged in server.log during shutdown if an open websocket session exists. Frequency is rare and not reproducible.

14:25:50,082 ERROR [org.xnio.listener] (default I/O-4) XNIO001007: A channel event listener threw an exception: java.util.concurrent.RejectedExecutionException: XNIO007007: Thread is terminating
    at org.xnio.nio.WorkerThread.execute(WorkerThread.java:620)
    at io.undertow.websockets.jsr.UndertowSession$3.handleEvent(UndertowSession.java:396)
    at io.undertow.websockets.jsr.UndertowSession$3.handleEvent(UndertowSession.java:388)
    at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
    at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameCloseListener.handleEvent(AbstractFramedChannel.java:1081)
    at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameCloseListener$2.run(AbstractFramedChannel.java:1040)
    at io.undertow.server.protocol.framed.AbstractFramedChannel$1.run(AbstractFramedChannel.java:144)
    at org.xnio.nio.WorkerThread.safeRun(WorkerThread.java:612)
    at org.xnio.nio.WorkerThread.run(WorkerThread.java:479)