Crash in Models tab

To reproduce, remove local configuration (rm -r ~/.config/BornAgain), and run GUI:

• Choose any instrument
• Choose any sample from library
• Simulation > Run simulation
• Settings > activate "Model tech view"
• Models > JobModel, then click "Expand/collapse tree" causes fatal error with message

BornAgain encountered a fatal error and can't continue execution.

Diagnostics of error:
Dynamic exception type: Error
std::exception::what: MaterialUtils::findMaterial() -> Error. Can't find material with identifier '{0f912863-350c-4e66-a93d-c533632222bc}'.