Dropped WS sipstack connection does not terminate active call session

[roginvs]

1. Call somewhere from sipml5
2. Terminate WS tcp connection (I used "# tcpkill port 8089" on server side)
3. Call will be still active (incoming and outgoing audio continue to work)
4. Now it is impossible to terminate this call on sipml5 side (only way to do this is to reload webpage): .hangup() method raises "Stack not started" error