Promotion dialogue silently fails if channel isn't selected

[FletcherAU]

When the conversation selector isn't filled the promotion modal returns None for channel. This causes respond() to kick back an error.

The server responded with: {'ok': False, 'error': 'invalid_arguments', 'response_metadata': {'messages': ['[ERROR] missing required field: channel']}})

Instead the modal should kick back a validation error similar to updateData