WebSocket sendMessage failed: Error: WebSocket is not open: readyState 3 (CLOSED)

STR:
- Install virtual things add-on on gateway built from master branch
- Click the + button on the Things screen
- Add some virtual Things

Expected:
- No errors

Actual:
- Things added successfully but errors on the console

```
ERROR  : WebSocket sendMessage failed: Error: WebSocket is not open: readyState 3 (CLOSED)
```