Simplecoapclient crashing when Discover/Observe/Cancel Observe

How to reproduce:

• Enter coap.me as Host
• Press "Discover" button
• Press "Observe" button for  default "/test" resource
• Press "Cancel Observe"

The crash below can be observed when pressing "Cancel Observe".

Thread 1 "simplecoapclien" received signal SIGSEGV, Segmentation fault.
QUrl::isEmpty (this=this@entry=0x7fffffffcad0) at io/qurl.cpp:1896
1896 return d->isEmpty();
(gdb) bt
#0 QUrl::isEmpty (this=this@entry=0x7fffffffcad0) at io/qurl.cpp:1896
#1 0x00007ffff672e36f in QCoapRequest::adjustedUrl (url=..., secure=<optimized out>) at qcoaprequest.cpp:305
#2 0x00007ffff6710755 in QCoapClient::cancelObserve (this=<optimized out>, url=...) at qcoapclient.cpp:506
#3 0x000055555555b663 in MainWindow::<lambda()>::operator() (__closure=0x7fffe400a0d0) at mainwindow.cpp:219
#4 QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, MainWindow::on_observeButton_clicked()::<lambda()> >::call (arg=<optimized out>, f=...)
at /home/ablasche/dev/qt/qt513/qtbase/include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:146
#5 QtPrivate::Functor<MainWindow::on_observeButton_clicked()::<lambda()>, 0>::call<QtPrivate::List<>, void> (
arg=<optimized out>, f=...)
at /home/ablasche/dev/qt/qt513/qtbase/include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:256
#6 QtPrivate::QFunctorSlotObject<MainWindow::on_observeButton_clicked()::<lambda()>, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void **, bool *) (which=<optimized out>, this_=0x7fffe400a0c0, r=<optimized out>,
a=<optimized out>, ret=<optimized out>)
at /home/ablasche/dev/qt/qt513/qtbase/include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:439
#7 0x00007ffff61e60fa in QtPrivate::QSlotObjectBase::call (a=0x7fffffffccc0, r=0x7fffffffda90, this=<optimized out>)
at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:394
#8 QMetaObject::activate (sender=sender@entry=0x5555558ad4c0, signalOffset=<optimized out>,
local_signal_index=local_signal_index@entry=2, argv=argv@entry=0x7fffffffccc0) at kernel/qobject.cpp:3782
#9 0x00007ffff61e65a1 in QMetaObject::activate (sender=sender@entry=0x5555558ad4c0,
m=m@entry=0x7ffff7daa4e0 <QAbstractButton::staticMetaObject>, local_signal_index=local_signal_index@entry=2,
argv=argv@entry=0x7fffffffccc0) at kernel/qobject.cpp:3652
#10 0x00007ffff7750706 in QAbstractButton::clicked (this=this@entry=0x5555558ad4c0, _t1=<optimized out>)
at .moc/moc_qabstractbutton.cpp:313
#11 0x00007ffff775094e in QAbstractButtonPrivate::emitClicked (this=this@entry=0x5555558acc80) at widgets/qabstractbutton.cpp:414
#12 0x00007ffff7751fb8 in QAbstractButtonPrivate::click (this=this@entry=0x5555558acc80) at widgets/qabstractbutton.cpp:407
#13 0x00007ffff7752272 in QAbstractButton::mouseReleaseEvent (this=0x5555558ad4c0, e=0x7fffffffd210)
at widgets/qabstractbutton.cpp:1011
#14 0x00007ffff7692bd4 in QWidget::event (this=0x5555558ad4c0, event=0x7fffffffd210) at kernel/qwidget.cpp:8939
#15 0x00007ffff7753463 in QAbstractButton::event (this=this@entry=0x5555558ad4c0, e=e@entry=0x7fffffffd210)
at widgets/qabstractbutton.cpp:968
#16 0x00007ffff780031b in QPushButton::event (this=0x5555558ad4c0, e=0x7fffffffd210) at widgets/qpushbutton.cpp:684
#17 0x00007ffff7648fc3 in QApplicationPrivate::notify_helper (this=this@entry=0x555555783220,
receiver=receiver@entry=0x5555558ad4c0, e=0x7fffffffd210) at kernel/qapplication.cpp:3739
#18 0x00007ffff7650921 in QApplication::notify (this=<optimized out>, receiver=0x5555558ad4c0, e=0x7fffffffd210)
at kernel/qapplication.cpp:3200
#19 0x00007ffff61b1f2b in QCoreApplication::notifyInternal2 (receiver=0x5555558ad4c0, event=0x7fffffffd210)
at kernel/qcoreapplication.cpp:1060
#20 0x00007ffff61b21ac in QCoreApplication::sendSpontaneousEvent (receiver=receiver@entry=0x5555558ad4c0,
event=event@entry=0x7fffffffd210) at kernel/qcoreapplication.cpp:1469
#21 0x00007ffff764fbe8 in QApplicationPrivate::sendMouseEvent (receiver=receiver@entry=0x5555558ad4c0,
event=event@entry=0x7fffffffd210, alienWidget=alienWidget@entry=0x5555558ad4c0, nativeWidget=0x7fffffffda90,
buttonDown=buttonDown@entry=0x7ffff7dd4960 <qt_button_down>, lastMouseReceiver=..., spontaneous=true,
onlyDispatchEnterLeave=false) at kernel/qapplication.cpp:2686
#22 0x00007ffff76b068a in QWidgetWindow::handleMouseEvent (this=this@entry=0x55555584ec70, event=event@entry=0x7fffffffd650)
at kernel/qwidgetwindow.cpp:662
#23 0x00007ffff76b2a7a in QWidgetWindow::event (this=0x55555584ec70, event=0x7fffffffd650) at kernel/qwidgetwindow.cpp:281
#24 0x00007ffff7648fc3 in QApplicationPrivate::notify_helper (this=this@entry=0x555555783220,
receiver=receiver@entry=0x55555584ec70, e=e@entry=0x7fffffffd650) at kernel/qapplication.cpp:3739
#25 0x00007ffff765065a in QApplication::notify (this=0x7fffffffda80, receiver=0x55555584ec70, e=0x7fffffffd650)
at kernel/qapplication.cpp:3096
#26 0x00007ffff61b1f2b in QCoreApplication::notifyInternal2 (receiver=0x55555584ec70, event=0x7fffffffd650)
at kernel/qcoreapplication.cpp:1060
#27 0x00007ffff61b21ac in QCoreApplication::sendSpontaneousEvent (receiver=receiver@entry=0x55555584ec70,
event=event@entry=0x7fffffffd650) at kernel/qcoreapplication.cpp:1469
#28 0x00007ffff6e4377d in QGuiApplicationPrivate::processMouseEvent (e=0x55555585ef90) at kernel/qguiapplication.cpp:2106
#29 0x00007ffff6e44c62 in QGuiApplicationPrivate::processWindowSystemEvent (e=e@entry=0x55555585ef90)
at kernel/qguiapplication.cpp:1841
#30 0x00007ffff6e1d50d in QWindowSystemInterface::sendWindowSystemEvents (flags=flags@entry=...)
at kernel/qwindowsysteminterface.cpp:1087
#31 0x00007fffefaac7fa in xcbSourceDispatch (source=<optimized out>) at qxcbeventdispatcher.cpp:105
#32 0x00007ffff3939387 in g_main_context_dispatch () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#33 0x00007ffff39395c0 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#34 0x00007ffff393964c in g_main_context_iteration () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#35 0x00007ffff621364a in QEventDispatcherGlib::processEvents (this=0x5555557fee10, flags=...)
at kernel/qeventdispatcher_glib.cpp:423
#36 0x00007fffefaacb68 in QXcbGlibEventDispatcher::processEvents (this=<optimized out>, flags=...) at qxcbeventdispatcher.cpp:143
#37 0x00007ffff61afc89 in QEventLoop::processEvents (this=this@entry=0x7fffffffda00, flags=..., flags@entry=...)
at kernel/qeventloop.cpp:138
#38 0x00007ffff61b00d7 in QEventLoop::exec (this=this@entry=0x7fffffffda00, flags=flags@entry=...) at kernel/qeventloop.cpp:225
#39 0x00007ffff61ba1b8 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1370
#40 0x00007ffff6e39ff6 in QGuiApplication::exec () at kernel/qguiapplication.cpp:1783
--Type <return> to continue, or q <return> to quit--
#41 0x00007ffff7648ea3 in QApplication::exec () at kernel/qapplication.cpp:2896
#42 0x000055555555b5ff in main (argc=<optimized out>, argv=<optimized out>) at main.cpp:61

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