Fix: Toggling between different namespaces crashes the viewer

[jkeshav-bvignesh]

This is just a simple variable name change that fixes the below error that occurs whenever we try to switch between various namespaces in the viewer. This is a common scenario when we have multiple trees running. The viewer will always crash when we try to toggle between these trees.

The error:

File "/path/to/py_trees_ros_viewer/backend.py", line 299, in connect
console.logdebug("cancelling existing snapshot stream connection [{}] [backend]".format(self.snapshot_stream_watcher))
AttributeError:  'Backend' object has no attribute 'snapshot_stream_watcher'

On further examination, it looks like snapshot_stream_watcher is referenced only in this particular line. The same function proceeds to assign a new snapshot to the variable self.snapshot_stream. This must have been missed out in a refactor.
Hope this helps!

Thank you @stonier for building and maintaining py_trees and all its associated tools & libraries!