Crash when accessing a null PcpNodeRef object through python

[marktucker]

Description of Issue

There is no protection against null pointers in the Pcp.NodeRef implementation, making it easy to cause a crash with python code. Even worse, there is not any way to detect a null pointer in the python code, so there is no way to avoid the crash even if you are aware of the potential danger.

Steps to Reproduce

1. Run the following code in a python shell with USD libraries available:

from pxr import Usd, UsdGeom
s = Usd.Stage.CreateInMemory()
p = UsdGeom.Xform.Define(s, "/foo").GetPrim()
n = p.GetAttributes()[0].GetResolveInfo().GetNode()

2. At this point, n refers to a null PcpNodeRef. But bool(n) returns True and n.layerStack will crash the process.

System Information (OS, Hardware)

Windows, Linux

Package Versions

23.08+, but I believe this has always been the case.

[jesschimein]

Filed as internal issue #USD-9288