Channel Manager - Add channel dialog can be opened when no blueprints are available

When no blueprints are available, the 'Add channel' dialog won't do anything. Still, it is possible to open it via the 'Add channel' button in the channel manager.
Instead, the button should not be shown at all.

Steps to reproduce:
1. remove all blueprints (e.g. remove the node /hst:hst/hst:blueprints)
2. open the channel manager

Expected behavior: no 'Add channel' button is visible
Actual behavior: the 'Add channel' button opens an empty dialog without any functionality