Post processing silhouette and model's silhouette cannot exist at the same time

Sandcastle: [example](https://sandcastle.cesium.com/#c=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)

Browser: Edge 91

Operating System: Windows 10 Pro

Steps to reproduce:
- Open sandcastle example
- Click the car (crash)
- Rerun the example
- Uncheck post processing silhouette;
- Click the car (everything is fine)


Start with the linked Sandcastle example and reproduce the crash by selecting the car while post-processing and model silhouettes are both enabled. Compare it with the working case where post-processing silhouette is unchecked; done means the example no longer crashes with both silhouette options active.

Written by the indexing model from the issue text.