This case was not properly handled by the previous fix in #107621.
The viewer node path function still assumes that the node editor space
has a valid ID pointer (`snode.id`) and that it is an `Object`.
The ID pointer can be null when setting a pinned node tree through the
python API, like so:
```Py
space.pin = True
space.node_tree = some_other_node_tree
```
In this case the `ED_node_tree_start` function is called without and ID
pointer, which is allowed! This patch makes sure that the viewer node
path function is only used with a valid object context.
Pull Request: https://projects.blender.org/blender/blender/pulls/107737
967cd7f99e