[Engine] Fixed NullReferenceException thrown in ChatModel.Close()

This can happen if no messages have ever been added to the ChatModel
Merge pull request #112 from oli-obk/fix/chatmodel_close
[Engine-XMPP] prevent Threaded parts from accessing XmppProtocolManager without lock or after Disposed has been called
[Engine-XMPP] not yet synced chats do not need to be disabled
[Engine-XMPP] removed unreachable code

ContactChat is already disabled with all the other chats, therefore IsEnabled is false.
Merge pull request #113 from oli-obk/fix/xmpp_threads
Merge branch 'stable'
Merge remote-tracking branch 'github/master'