I've tried a few examples and some of them are crashing (e.g. examples/widgets/mainwindows/application).
From what I've seen it seems that (sometimes) gcc5 fails to static_cast QCoreAppication to QGuiApplication, qGuiApp macro (static_cast<QGuiApplication *>(QCoreApplication::instance())) is 0.
I realy have no idea why somethimes qGuiApp works and sometime doesn't, and I have no idea if its gcc's fault or Qt's.
I attached the full backtrace.