[Storm] Crash on material changes when material is using primvar

[jufrantz]

Description of Issue

Since USD 20.05, we get a seg fault when attempting to change material assignments from a scene delegate in some cases. It happens when the currently bound material is using uniform primvar(s) but not the new one, when the list of required elements buffers becomes empty at draw item update.

I reproduced the issue with usdview by turning off the enable scene materials setting.

Steps to Reproduce

1. Run testusdview with python script and usd scene in reproUsdViewCrashOnMaterialChange.zip
   testusdview --renderer GL --testScript reproUsdViewCrashOnMaterialChange.py reproUsdViewCrashOnMaterialChange.usda

2. Notice the crash

System Information (OS, Hardware)

Linux

Package Versions

USD 20.05

[jilliene]

Filed as internal issue #USD-6022

[jufrantz]

Hi,
Just tested the fix on our side with USD 21.02 and it works like a charm.
Thank you !