[BUG] Duplicated sub in Waveform widget for asynchronous readback

```python
 Attempted to create duplicate stream subscription for topics=EndpointInfo(endpoint='info/devices/async_readback/94c08639-a345-4681-8323-90f1a35812f0/waveform', message_type=<class 'bec_lib.messages.DeviceMessage'>, message_op=<MessageOp.STREAM: ['xadd', 'xrange', 'xread', 'register_stream', 'keys', 'get_last', 'delete']>)
```