Thanks for the report, Alam!
I can confirm this. Here's the stack trace:

====================
signon: browser-request.cpp 585 buildDialog Dialog was built
signon: request.cpp 137 setWindow Requesting window embedding

Thread 1 "signon-ui" received signal SIGSEGV, Segmentation fault.
QWindow::show (this=0x0) at kernel/qwindow.cpp:1851
1851	kernel/qwindow.cpp: No such file or directory.
(gdb) bt
#0  0x00007ffff40bf8de in QWindow::show() (this=0x0) at kernel/qwindow.cpp:1851
#1  0x000055555558b5e2 in SignOnUi::RequestPrivate::setWindow(QWindow*) (this=this@entry=0x5555558d0070, window=0x0) at request.cpp:139
#2  0x000055555558b7e0 in SignOnUi::Request::setWidget(QWidget*) (this=<optimized out>, widget=0x5555558d07f0) at request.cpp:308
#3  0x000055555556c25b in SignOnUi::BrowserRequestPrivate::start() (this=0x5555558d01f0) at browser-request.cpp:599
#4  0x000055555558d794 in SignOnUi::ServicePrivate::runQueue(QQueue<SignOnUi::Request*>&) (this=this@entry=0x5555558ba200, queue=...) at service.cpp:141
#5  0x000055555558da62 in SignOnUi::ServicePrivate::enqueue(SignOnUi::Request*) (this=this@entry=0x5555558ba200, request=request@entry=0x5555558d03a0) at service.cpp:126
#6  0x000055555558def0 in SignOnUi::Service::queryDialog(QMap<QString, QVariant> const&) (this=0x5555558ba790, parameters=...) at service.cpp:227
#7  0x0000555555593571 in SignOnUi::Service::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (_o=<optimized out>, _c=<optimized out>, _id=<optimized out>, _a=0x7fffffffd360) at moc_service.cpp:107
#8  0x00005555555936d8 in SignOnUi::Service::qt_metacall(QMetaObject::Call, int, void**) (this=0x5555558ba790, _c=QMetaObject::InvokeMetaMethod, _id=1, _a=0x7fffffffd360)
    at moc_service.cpp:169
#9  0x00007ffff3d56650 in  () at /usr/lib/x86_64-linux-gnu/libQt5DBus.so.5
#10 0x00007ffff3d5a4fd in  () at /usr/lib/x86_64-linux-gnu/libQt5DBus.so.5
#11 0x00007ffff3d5aadc in  () at /usr/lib/x86_64-linux-gnu/libQt5DBus.so.5
#12 0x00007ffff3d5d5ae in  () at /usr/lib/x86_64-linux-gnu/libQt5DBus.so.5
#13 0x00007ffff21bb359 in QObject::event(QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#14 0x00007ffff484835c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#15 0x00007ffff484fb11 in QApplication::notify(QObject*, QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5
#16 0x00007ffff218e8a0 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#17 0x00007ffff219102d in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#18 0x00007ffff21e2b03 in  () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#19 0x00007ffff2ca2377 in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#20 0x00007ffff2ca25e0 in  () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#21 0x00007ffff2ca268c in g_main_context_iteration () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#22 0x00007ffff21e2f0f in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#23 0x00007ffff218c88a in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#24 0x00007ffff2194ffc in QCoreApplication::exec() () at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#25 0x0000555555566b03 in main(int, char**) (argc=<optimized out>, argv=<optimized out>) at main.cpp:99
====================