Error using setSheenColorFactor in Material

Hi,

The issue is reproducible from model-viewer version 3.2.0 to 4.1.0:

```js
const viewer = document.getElementsByTagName('model-viewer')[0]
viewer.model.materials[0].setSheenColorFactor('#ffffff')
```

Output:
```
Uncaught TypeError: Cannot read properties of undefined (reading 'set')
```