Resolve "fix(viz): SightViewer organ manager material selector crashes when no material is found"

Description

The material selector of sight_viewer now works again, allowing to change between three different materials.

How to test it?

• Launch SightViewer.
• Load an organ, anything.
• Go to the organ manager.
• Select the organ you just loaded.
• Scroll back to Material selector.
• Select any material between Default, OrenNayer or Foggy
• Observe the material is applied
• Click Reload.
• Nothing bad happens

Decreased code coverage

If you reduce test coverage, it is mandatory to justify it here

Related issue(s)

Closes #802 (closed)