Crash when unmuting layer

[marktucker]

Description of Issue

It is possible to cause a crash when unmuting a layer on a stage, generating the error message "attempted member lookup on NULL TfWeakPtr".

Steps to Reproduce

1. Unzip the attached set of simple usda files. The content is not important, but the composition hierarchy does seem relevant (root layer with a sublayer that payloads a layer with a sublayer).
   unmute_crash.zip
2. Run python in the directory containing the unzipped usda files. Run the following python code:
   from pxr import Usd
   import os
   s = Usd.Stage.CreateInMemory()
   r = s.GetRootLayer()
   r.subLayerPaths.append('d.usda')
   s2 = Usd.Stage.CreateInMemory()
   r2 = s2.GetRootLayer()
   r2.subLayerPaths.append('d.usda')
   s.MuteAndUnmuteLayers([os.getcwd() + '/c.usda'], [])
   s2.MuteAndUnmuteLayers([os.getcwd() + '/c.usda'], [])
   r.subLayerPaths.clear()
   r.subLayerPaths.append('e.usda')
   s.MuteAndUnmuteLayers([], [os.getcwd() + '/c.usda'])
3. Executing the last line should generate the message: "attempted member lookup on NULL TfWeakPtr", followed by a crash.

Package Versions

19.07

[jilliene]

Filed as internal issue #USD-5386