QQmlDebugServer::instance() crashes if qApp is 0

When running the following test app

https://github.com/pyinstaller/pyinstaller/blob/develop/tests/interactive/test_pyqt5_qml.py

I get a crash (stack trace attached) when the app is frozen using PyInstaller, but not when running it directly.

The issue appears to be that in QQmlDebugServer::instance() the call to qApp (which seems to be a macro for QCoreApplication::instance(), line 273 qqmldebugserver.cpp) returns 0 and this is not checked.

A possible fix is to check this return value and return 0 for QQmlDebugServer::instance() if it is 0.