Android NativeCall demo got "disconnected from server"

I built the android NativeCall example project (running on real device Android 4.0.4 / API 15,) after clicking Join to the demo.openwebrtc.org, I failed with "Disconnected from server", apparently reaching the callback method:

``` java
@Override
public void onDisconnect() {
Toast.makeText(this, "Disconnected from server", Toast.LENGTH_LONG).show();
updateVideoView(false);
mStreamSet = null;
if (mRtcSession != null) { // I also encountered an NPE here if not protected by the condition
mRtcSession.stop();
mRtcSession = null;
}
mSignalingChannel = null;
}
```

Any clue?