Ensure hasTopLevelWidgets exists before calling it

[macevhicz]

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)

Proposed Changes

The QApplication.instance().topLevelWidgets method does not exist in Qt6 when running headless, so confirm it exists before attempting to call it.
QApplication.instance() returns a QGuiApplication instance. It's unclear if this is the new behavior, or if it's that Qt6 removed the topLevelWidgets method from QGuiApplication.