Comment 4 for bug 1923053

I'm having this issue as well. Here's a backtrace from GDB, with some debug symbols:

(gdb) bt
#0 doActivate<false> (sender=0x0, signal_index=9, argv=argv@entry=0x7fffffffb9a0) at kernel/qobject.cpp:3768
#1 0x00007fffeafb5cb8 in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7fffebed30a0 <QGuiApplication::staticMetaObject>, local_signal_index=local_signal_index@entry=1,
    argv=argv@entry=0x7fffffffb9a0) at kernel/qobject.cpp:3946
#2 0x00007fffeb9515a6 in QGuiApplication::screenAdded (this=<optimized out>, _t1=<optimized out>, _t1@entry=0x5555558b3cf0) at .moc/moc_qguiapplication.cpp:389
#3 0x00007fffeb936d80 in QWindowSystemInterface::handleScreenAdded (ps=ps@entry=0x5555558a5850, isPrimary=<optimized out>) at kernel/qwindowsysteminterface.cpp:826
#4 0x00007fffdef2d540 in QXcbConnection::initializeScreens (this=this@entry=0x555555893950) at qxcbconnection_screens.cpp:413
#5 0x00007fffdef082a4 in QXcbConnection::QXcbConnection (this=0x555555893950, nativeInterface=<optimized out>, canGrabServer=<optimized out>, defaultVisualId=<optimized out>, displayName=<optimized out>)
    at qxcbconnection.cpp:103
#6 0x00007fffdef0b10f in QXcbIntegration::QXcbIntegration (this=0x555555893830, parameters=..., argc=<optimized out>, argv=<optimized out>) at qxcbintegration.cpp:197
#7 0x00007ffff7fb9593 in QXcbIntegrationPlugin::create (this=<optimized out>, system=..., argv=0x7fffffffd868, argc=@0x7fffffffd60c: 1, parameters=...) at qxcbmain.cpp:56
#8 QXcbIntegrationPlugin::create (this=<optimized out>, system=..., parameters=..., argc=@0x7fffffffd60c: 1, argv=0x7fffffffd868) at qxcbmain.cpp:53
#9 0x00007fffeb9559fc in init_platform (argv=<optimized out>, argc=@0x7fffffffd60c: 1, platformThemeName=..., platformPluginPath=..., pluginNamesWithArguments=...) at kernel/qguiapplication.cpp:1223
#10 QGuiApplicationPrivate::createPlatformIntegration (this=0x555555887e20) at kernel/qguiapplication.cpp:1481
#11 0x00007fffeb956e88 in QGuiApplicationPrivate::createEventDispatcher (this=<optimized out>) at kernel/qguiapplication.cpp:1498
#12 0x00007fffeaf8be46 in QCoreApplicationPrivate::init (this=this@entry=0x555555887e20) at kernel/qcoreapplication.cpp:834
#13 0x00007fffeb959e02 in QGuiApplicationPrivate::init (this=0x555555887e20) at kernel/qguiapplication.cpp:1527
#14 0x00007fffef2f5fed in QApplicationPrivate::init() () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#15 0x000055555571c68b in SharedTools::QtSingleApplication::QtSingleApplication (this=<optimized out>, appId=..., argc=<optimized out>, argv=<optimized out>, this=<optimized out>, appId=...,
    argc=<optimized out>, argv=<optimized out>) at ./src/3rdparty/qtsingleapplication/qtsingleapplication.cpp:56
#16 0x000055555563d30c in OCC::Application::Application (this=<optimized out>, argc=<optimized out>, argv=<optimized out>, this=<optimized out>, argc=<optimized out>, argv=<optimized out>)
    at ./src/gui/application.cpp:116
#17 0x00005555556093c7 in main (argc=<optimized out>, argv=0x7fffffffd868) at ./src/gui/main.cpp:80

As you can see, the "sender" argument to qobject.cpp's doActivate function is NULL, which it's not supposed to be. The first line of that function is line 3768, and that line is "QObjectPrivate *sp = QObjectPrivate::get(sender);". Since "QObjectPrivate::get" dereferences its argument, that's obviously a segfault.

From stuff I'm reading online, it seems like this might be triggered by compiling with LTO. That's sort of just a wild guess though.