stop() and open() hangs when flexsim windows has been closed

[mylastresort]

Problem:

When a FlexSim instance is launched via fp.launch(...), closing the FlexSim window does not raise an exception to the Python caller. Instead, it simply hangs.

This makes it impossible to detect when a user closes the FlexSim window and handle it gracefully.

Testing:

1. Launch a FlexSim instance with GUI enabled
2. Close the FlexSim window manually
3. Call any controller method controller.open(...) or controller.stop(...)
4. Verify that RuntimeError is raised with message "FlexSim instance is not running"