Smack - Thread starting during runtime shutdown

Hi, I just received one strange error message from crashlytics. Is it any known bug? I'm using Smack version 4.1.0-alpha5. Thanks!

java.lang.InternalError: Thread starting during runtime shutdown       at java.lang.Thread.nativeCreate(Thread.java)       at java.lang.Thread.start(Thread.java:1063)       at org.jivesoftware.smack.util.Async.go(Async.java:29)       at org.jivesoftware.smack.AbstractXMPPConnection.firePacketSendingListeners(AbstractXMPPConnection.java:735)       at org.jivesoftware.smack.tcp.XMPPTCPConnection.access$3300(XMPPTCPConnection.java:135)       at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketWriter.writePackets(XMPPTCPConnection.java:1397)       at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketWriter.access$2800(XMPPTCPConnection.java:1242)       at org.jivesoftware.smack.tcp.XMPPTCPConnection$PacketWriter$1.run(XMPPTCPConnection.java:1281)